Memory comes in three popular forms: RAM chips, ROM chips, and disks.

You already learned about RAM chips and ROM chips. Let's examine disks.

A computer disk is round, like a phonograph record.

Three kinds

You can buy three popular kinds of computer disks:
A floppy disk is made of flimsy material. It's permanently encased in a sturdy, square dust jacket.

A hard disk is made of firmer material. It typically hides in your computer permanently, unseen.

A CD-ROM is a compact disk. It's the same kind of CD compact disk that plays music.

Each kind has its own advantages and disadvantages.

Floppy disks are the cheapest (under 50 per disk) and the easiest to mail to your friends: just stick the floppy disk in an envelope, perhaps with some padding. Unfortunately, floppy disks work the most slowly, and they hold the least data: the typical floppy disk holds about 1 megabyte, while the typical hard disk or CD-ROM can hold many hundreds of megabytes.

Hard disks work the fastest - over 20 times faster than the other kinds! But hard disks are also the most expensive. Moreover, they typically can't be removed from your computer and therefore can't be mailed to your friends.

CD-ROMs are the best value: CD-ROM disks cost less than 1 per megabyte to manufacture. But they have a frustrating limitation: the information on CD-ROM disks cannot be edited.

Since each kind of disk has its own advantages and disadvantages, you'll want to buy all three kinds.


Computer experts argue about spelling. Some experts write "disk", others write "disc".

Most manufacturers write "disk" when referring to floppy disks or hard disks, but write "disc" when referring to CD-ROMs. That inconsistency annoys me.

To be more consistent, I'll always write "disk", even when referring to CD-ROMs. Most computer magazines (such as PC Magazine and PC World) feel the same way I do: they always write "disk". The growing tendency is to always write "disk".

For hard disks, IBM used to write "disc" but now writes "disk".

Floppy disks

A floppy disk (or diskette) is round but comes permanently sealed in a square dust jacket. (Don't try to remove the floppy disk from its square jacket.)

The floppy disk is as thin and flimsy as a sheet of paper but is protected by the sturdy, square jacket that encases it.

Three sizes

Floppy disks come in three sizes.

The most popular size is called a 3 1/2-inch floppy disk, because it comes in a square jacket that's about 3 1/2 inches on each side. (Actually, each side of the jacket is slightly more than 31/2 inches, and the disk's diameter is slightly less.)

An older size, used mainly on older computers, is called 5 1/4-inch. It comes in square jacket that's exactly 5 1/4 inches on each side.

An even older size, 8-inch, is used just on ancient computers that are no longer built.

Those three sizes have nicknames:
An 8-inch floppy disk is called a large floppy.
A 51/4-inch floppy disk is called a minifloppy.
A 31/2-inch floppy disk is called a microfloppy.

Jacket colors

The jacket of a 5 1/4-inch or 8-inch floppy disk is usually black. The jacket of a 31/2-inch floppy disk is usually black, blue, white, or beige (very light grayish brown).

If you pay a surcharge, you can get jackets that have wilder colors.


8-inch floppies were invented in the early 1970's by IBM. 5 1/4-inch floppies were invented in the late 1970's by Shugart Associates, which later became part of Xerox.

3 1/2-inch floppies were invented in the 1980's by Sony. They've become the most popular size because they're the smallest, cutest, and sturdiest. They're small enough to fit in the pocket of your shirt, cute enough to impress your friends, and sturdy enough to survive when you fall on your face. They're also easy to mail, since they're small enough to fit in a standard white business envelope and sturdy enough to survive the U.S. Postal System. Yup, nice things come in small packages!

Magnetized iron

The round disk (which hides inside the square jacket) is coated with rust, so it looks brown. Since the rust is made of iron, which can be magnetized, the disk stores magnetic signals. The pattern of magnetic signals is a code representing your data.


To use a floppy disk, you must buy a floppy-disk drive, which is a computerized record player.

If the drive is external, it's a box sitting near the computer. If the drive is internal, it's built into the middle of the computer.

The drive has a slit in its front side. To use the drive, push the disk (including its jacket) into the slit.

When pushing the sheathed treasure into the box's slit, don't shove too hard. Oooh! Please be gentle!

When you push your disk into the slit, don't push the disk in backwards or upside-down! Here's how to push the disk in correctly:
First, notice that the disk's jacket has a label on it and also has a big oval cutout. (If the disk is 3 1/2-inch, the cutout is covered by a metal slider.) Insert the disk so that the oval cutout goes into the drive before the label does. If the drive's slit is horizontal, make sure the label is on the top side of the jacket; if the drive is vertical, make sure the label is on the left side of the jacket.

After putting the disk into the slit, close the latch to cover the slit. (If the disk is 3 1/2-inch, there is no latch.) Since the slit and latch act as a door, closing the latch is called closing the door.

As soon as you close the door, the disk drive automatically positions the disk onto the turntable that's hidden inside the drive. The turntable's called the spindle. It can spin the disk quickly.

Like a record player, the disk drive contains an arm with a "needle" on it. The needle is called the read-write head, because it can read what's on the disk and also write new information onto the disk.

Here's how to write new information onto the disk. Put your fingers on the computer's keyboard. Type a command that tells the computer you want to use the disk. Then type the information you want to transfer to the disk.

To transfer the information to the disk, the computer lowers the read-write head onto the disk. An electrical charge passes through the head. The charge creates an electromagnetic field, which magnetizes the iron on the disk's surface. Each iron particle has its own north and south pole; the patterns formed by the north and south poles are a code that stands for the information you're storing.

As the disk spins, the head remains stationary, so that the head draws a circle on the spinning disk's surface. The circle's called a track. To draw the circle, the head doesn't use ink; instead, it uses a pattern of magnetic pulses. Since your eye can't see magnetism, your eye can't see the circle; but it's there!

When you start using a blank disk, the arm puts the head near the disk's outer rim, so that the head's track (circle) is almost as wide as the disk. That track's called track 0.

Then the arm lifts the head, moves the head slightly closer to the virgin disk's center, and puts the head back down onto the disk again. The head draws another circular track on the disk, but this new circular track is slightly smaller than the previous one. It's called track 1.

Then the head draws track 2, then track 3, then track 4, and so on, until the head gets near the center of the disk, and draws the last circular track (which is smaller than the other tracks).

To organize the information on a track, the computer divides the track into sectors. Each "sector" is an arc of the circle.

Single-sided versus double-sided drives
A modern disk drive has two read-write heads. One head uses the disk's top surface, while the other head uses the disk's bottom, so that the drive can use both sides of the disk simultaneously. That's called a double-sided disk drive. The drive puts information onto the disk by first using track 0 of the main side, then track 0 of the flip side, then track 1 of the main side, then track 1 of the flip side, etc.

If a disk drive is not modern - if it's ancient and primitive - it has just one read-write head, which uses just one side of the disk. The flip side of the disk is unused. That kind of drive is called a single-sided disk drive. Which side of the disk does the drive use? Though some drives use the side that has the label, other drives (by other manufacturers) use the side opposite the label instead.

Double-sided is also called DS and 2-sided and 2S. Single-sided is also called SS and 1-sided and 1S.

How many kilobytes can you fit on a floppy disk? The answer depends on which kind of drive you have.

The most popular kind of drive is called a 3 1/2-inch high-density floppy drive. Here's how it works:
It holds a 3 1/2-inch floppy disk. It writes on both sides of the disk simultaneously, since it's a double-sided disk drive. It writes 80 tracks on each side. It divides each track into 18 sectors. Each sector holds "512 bytes", which is half a kilobyte, 1/2K.

Since the disk has 2 sides, 80 tracks per side, 18 sectors per track, and 1/2K per sector, the disk's total capacity is "2 times 80 times 18 times 1/2K", which is 1440K. So altogether, the disk holds 1440K. That's called 1.44M (where an M is defined as being 1000K). That's why a 3 1/2-inch high-density floppy drive is also called a 1.44M drive.

The kind of disk you put into it is called a 1.44M floppy disk (or a 3 1/2-inch high-density floppy disk). Since the disk holds 1.44M (which is 1440K), and since a K is 1024 bytes, the disk holds "1440 times 1024" bytes, which is 1,474,560 bytes altogether. That's a lot of bytes!

Although the disk holds 1440K, some of those K are used for "bureaucratic overhead" (such as holding a directory that reminds the computer which data is where on your disk). A Mac uses just 1 sector (1/2K) for bureaucratic overhead. An IBM-compatible computer uses 33 sectors (16 1/2K) for bureaucratic overhead, leaving just 14231/2K (1,457,664 bytes) for your data.

When you buy a blank disk to put in a 1.44M drive, make sure the disk is the right kind. Make sure the disk is 3 1/2-inch; and to get full use of what the drive can accomplish, make sure the disk is high-density! The abbreviation for "high-density" is . A high-density 3 1/2-inch disk has the letters HD stamped in white on its jacket; but the H overlaps the D, so it looks like this: HD. Also, a high-density 31/2-inch disk has an extra square hole cut through its jacket.

Old computers use inferior floppy drives, whose capacities are below 1.44M.
A capacity below 150K is called single-density (SD).
A capacity above 150M but below 1M is called double-density (DD).
A capacity above 1M is called high-density (HD).
Anything below high-density is called low-density.

Although the jacket of a high-density 31/2-inch disk has "HD" stamped on it and an extra hole punched through it, the jackets of other kinds of disks often lack any distinguishing marks. Too bad!

Popular IBM-compatible drives
For IBM-compatible computers, four kinds of floppy drives have been popular:

IBM drive's name Capacity Details
51/4-inch double-density 360K 40 tracks per side, 9 sectors per track
51/4-inch high-density 1200K (which is 1.2M) 80 tracks per side, 15 sectors per track
31/2-inch double-density 720K 80 tracks per side, 9 sectors per track
31/2-inch high-density 1440K (which is 1.44M) 80 tracks per side, 18 sectors per track
Each of those IBM-compatible drives is double-sided and has 1/2K per sector. They're manufactured by companies such as NEC , Teac , Chinon , Epson , and Alps . The fanciest drives (31/2-inch high-density) used to be expensive, but now you can buy them for just $29 from mail-order discount dealers (such as USA Flex at 444 Scott Dr., Bloomingdale IL 60108, phone 800-723-2261 or 708-582-6206).

Mac drives
For Mac computers, three kinds of floppy drives have been popular:

Mac drive's name Capacity Details
1-sided double-density 400K 1 side, 8-12 sectors per track
2-sided double-density 800K 2 sides, 8-12 sectors per track
high-density 1440K (which is 1.44M) 2 sides, 18 sectors per track
Each Mac drive is 31/2-inch and has 80 tracks per side, 1/2K per sector. The Mac's high-density drive is called the Mac Superdrive.

On a disk, the inner tracks have smaller diameters than the outer tracks. Most drives squeeze as many sectors onto an inner track as onto an outer track, but the Mac double-density drives puts fewer sectors onto the inner tracks and put extra sectors onto the outer tracks. Specifically, the outer 16 tracks are divided into 12 sectors, the next 16 tracks into 11 sectors, the next 16 into 10, the next 16 into 9, and the inner 16 into 8.

Drives for other computers
For other computers, many kinds of floppy drives have been invented:

Computer Drive capacity Details
Apple 2 family 140K 51/4", 1 side, 35 tracks, 16 sectors, 1/4K per sector
Tandy Color Computer 1571/2K 51/4", 1 side, 35 tracks, 18 sectors, 1/4K per sector
Tandy Models 3, 4, 4P 180K 51/4", 1 side, 40 tracks, 18 sectors, 1/4K per sector
Tandy Model 4D 360K 51/4", 2 sides, 40 tracks, 18 sectors, 1/4K per sector
Commodore 64 1703/4K 51/4", 1 side, 35 tracks, 17-21 sectors, 1/4K per sector
Commodore Amiga 880K 31/2", 2 sides, 80 tracks, 11 sectors, 1/2K per sector
For the Commodore 64, the 17 outer tracks are divided into 21 sectors, the next 7 tracks into 19 sectors, the next 6 tracks into 18 sectors, and the inner 5 tracks into 17 sectors.

In the disk drive, the disk spins quickly. The exact speed depends on what size disk the drive uses.
Low-density 51/4-inch disks revolve 5 times per second. That makes 300 revolutions per minute, 300 rpm.
8-inch disks and high-density 51/4-inch disks revolve faster: 6 times per second (360 rpm).
31/2-inch disks revolve even faster: between 61/2 and 10 times per second.

Buying disks

When you buy a floppy disk, make sure its size matches the size of the drive. For example, a 31/2-inch disk will not work in a 51/4-inch drive.

If you buy a blank 51/4-inch floppy disk, you can stick it into any normal 51/4-inch drive, regardless of who manufactured the drive and who manufactured the computer. But after you've put information onto the disk, that information is understandable only to your kind of computer. For example, an Apple 2e cannot understand what an IBM PC writes.

When you go into a computer store to buy a disk that contains software, tell the salesperson which kind of computer you have, so that the salesperson can give you a disk containing information understandable to your computer.

If your drive is single-density or double-density, it cannot handle high-density disks at all.

If your drive is 51/4-inch and high-density, it can read single-density and double-density disks, but it might have trouble writing new information onto them. So when buying blank disks for your 51/4-inch high-density drive to write on, avoid buying single-density or double-density disks.

The three crummy kinds of 51/4-inch floppy disks (single-sided single-density, single-sided double-density, and double-sided double-density) are all manufactured by the same process as each other. The only difference is the manufacturer's "guarantee": a double-sided double-density disk is "guaranteed" to work on both sides and hold lots of data; a single-sided or single-density disk is not. Even if you buy a disk that has a poor guarantee (just "single-sided single-density"), it typically works fine even if you use both sides and store lots of data. The only difference is that the manufacturer hasn't bothered testing the second side and hasn't bothered testing double-density data. During the 1970's and 1980's, single-sided single-density disks were significantly cheaper than double-sided double-density, but now the prices are about the same.

Formatting the disk
Before you can use a blank floppy disk, its surface must be formatted (divided into tracks and sectors). Buy a disk that's been formatted already, or buy an unformatted disk and format it by typing a command on your computer's keyboard.

After the disk's been formatted, you can store whatever information you wish onto the disk. Do not tell the drive to format that disk again. If you accidentally make the drive format the same disk again, the drive will create new tracks and sectors on the disk, and erase the old tracks and sectors, and therefore erase all your old data!

If a disk is blank, format it before you use it.
If a disk already contains info, do not format it; it's been formatted already.

Name brands
The most famous manufacturers of floppy disks are Verbatim and Maxell. But instead of buying those brands, buy generic floppy disks instead. The generics cost less and typically work just as well.

Discount dealers
To get the lowest prices on generic floppy disks, contact MEI Micro Center (1100 Steelwood Rd., Columbus OH 43212, 800-634-3478) or Diskettes Unlimited (6206 Long Dr., Houston TX 77087, 800-DOG-DISK).

For example, here are the prices from MEI Micro Center for double-sided disks:

Kind of disk 100 disks 1000 disks
51/4-inch double-density ,unformatted $19+$1.20 $160+$12
51/4-inch high-density, unformatted $26+$1.20 $210+$12
31/2-inch high-density, formatted $28+$2.40 $250+$24
Add up the prices of what you want, then add the handling charge ($3.25). For example, for 100 of the best disks (31/2-inch high-density, formatted), MEI charges you $28 (for the disks) + $2.40 (shipping) + $3.25 (handling), which is $33.65. That's about 34 per disk. For 1000 of the best disks, MEI charges you $250 + $24 + $3.25, which is $277.25, which comes to about 28 per disk. Diskettes Unlimited charges even less but might give you slightly lower quality; for details, phone them.

What's a disk worth?
Although you can buy a blank floppy disk for under 50, a disk containing information costs much more. The price depends on how valuable the information is. A disk that explains to the computer how to play a game costs about $25. A disk teaching the computer how to handle a general business task (such as accounting, filing, or correspondence) usually costs about $100.

A disk containing intimate, personal data about your business's customers, suppliers, employees, and methods is worth even more - perhaps thousands of dollars! To compute how much it's worth to you, imagine that you've lost it, or that it fell into the wrong hands!

Protect your disks

Most parts of a computer system are sturdy: even if you bang on the keyboard and rap your fist against the screen, you probably won't do any harm. Only one part of a computer system is delicate: that part is the disk. Unfortunately, the magnetic signals on your disk are easy to destroy.

One way to accidentally destroy them is to put your disk near a magnet; so keep your disks away from magnets! For example, keep your disk away from paper clips that have been in a magnetized paper-clip holder. Keep your disk away from speakers (such as the speakers in your stereo, TV, and phone), because all speakers contain magnets. Keep your disk away from electric motors, because motors generate an electromagnetic field. So to be safe, keep your disk at least six inches away from paper clips, stereos, TV's, telephones, and motors.

Keep your disk away from heat, because heat destroys the disk's magnetism and "melts" your data. So don't leave your disk in the hot sun; don't leave it on a sunny windowsill; don't leave it in the back of your car on a hot day. If your disk drive or computer feels hot, quickly lower the temperature, by getting an air conditioner or at least a fan.

31/2-inch floppy disks come in strong jackets, but 51/4-inch and 8-inch floppy disks come in jackets that are too weak and thin to protect disks from pressure. Don't squeeze your disk. Don't put it under a heavy object, such as a paperweight or a book. If you want to write a note on the disk's jacket, don't use a ball-point pen (which crushes the disk); use a soft felt-tip pen instead.

Keep the disk away from dust. For example, don't smoke cigarettes near the disk, because the smoke becomes dust that lands on the disk and wrecks the data.

Keep the disk dry. If you must transport a disk during a rainstorm, put the disk in a plastic bag. Never drink coffee or soda near the disk: your drink might spill.

To handle the disk, touch just the disk's jacket, not the brown disk itself. Holes in the jacket let you see the brown disk inside; don't put your fingers in the holes.

Power surges in ancient computers
If your computer's an IBM clone or by Apple, skip ahead to the next topic ("Write-protect notch").

If your computer is made by Commodore or Radio Shack and is so ancient that it's not an IBM PC clone, be careful: flipping the power switch on your ancient computer creates an electrical surge that wrecks the disk. On such a computer, don't flip the power switch when the drive contains a disk. Flip the power switch just when the drive's empty.

To turn such a computer on, make sure the drive's empty, then flip the power switch on. After the power's come on, insert the disk.

Before turning such a computer off, remove the disk from the drive. When the drive's empty, turn off the power.

Write-protect notch
When you buy a blank 51/4-inch or 8-inch floppy disk, the disk comes in a square black jacket. Since the jacket's square, it has four sides; but one of the sides has a notch cut into it.

You can cover the notch, by sticking a plastic tab over it. The tab has a gummed back, so you can stick it on the disk easily and cover the notch. You get the tab free when you buy the disk.

(For a 31/2-inch disk, the notch is different: it's a square hole near the jacket's corner but not on the jacket's edge. To cover it, you use a black slider instead of a tab. On old Apple Mac disks, the slider was red instead of black.)

Whenever you ask the computer to change the info on the disk, the drive checks whether you've covered the notch.

For a 51/4-inch disk, the normal situation is for the notch to be uncovered. For a 3 1/2-inch or 8-inch disk, the normal situation is for the notch to be covered.

If the situation's normal, the computer will obey your command: it will change the info on the disk as you wish. But if the situation's abnormal (because the notch is covered when it should be uncovered, or is uncovered when it should be covered), the computer will refuse to change the disk's info.

Suppose your disk contains valuable info, and you're afraid some idiot will accidentally erase or alter that info. To prevent such an accident, make the situation abnormal (by changing whether the notch is covered), so that the computer will refuse to change the disk's info. It will refuse to erase the disk; it will refuse to add new info to the disk; it will refuse to alter the disk; it will refuse to write onto the disk. The disk is protected from being changed; it's protected from being written on. The disk is write-protected (or locked).

Since the tab affects whether the disk is write-protected, the tab is called a write-protect tab, and the notch is called a write-protect notch.

When you buy a disk that already contains info, the disk usually comes write-protected, to protect you from accidentally erasing the info. So if you buy a 51/4-inch floppy disk that already contains info, it might come with a write-protect tab already covering the notch, to write-protect the disk.

Instead of creating a notch and then covering it with a tab, some manufacturers save money by getting special disks that have no notch. The computer treats a notchless disk the same way as a disk whose notch is covered.

Even if you handle your disk very carefully, eventually something will go wrong, and some of the info on your disk will get wrecked accidentally.

To prepare for that inevitable calamity, tell the computer to copy all info from the disk onto a blank disk, so that the blank disk becomes an exact copy of the original. Store the copy far away from the original: store it in another room, or - better yet - another building, or - better yet - another city. If you're working in a country that's having a war, store the copy in another country.

The copy is called a backup. Use the backup disk when the original disk gets wrecked.

Making a backup disk is like buying an insurance policy: it protects you against disasters.

Every evening, make backup copies of all your disks - except for disks containing the same info as the day before.

Each week, I get phone calls from distressed business executives whose disks got wrecked and who didn't make backups. All I can offer them is sympathy. Their companies are ruined. Remember: "a backup a day keeps disaster away!"

When you buy a floppy that already contains software, try copying the floppy before you begin using it. If you're lucky, the computer will make the backup copy without any hassles. If you're unlucky, the software company has put instructions on the floppy that make the computer refuse to copy the disk, because the company fears that you'll illegally give copies to all your friends for free. A floppy that the computer refuses to copy, and which is therefore protected against illegal copying, is called copy-protected. A floppy that you can copy is called copyable (or unprotected).

Short files
The information on the disk is divided into files. Each file has its own name and its own purpose. For example, one file might be named JILL and consist of a memo that you wrote to your friend Jill; another file might be named PAYROLL and consist of information about your company's payroll. Each file consists of many sectors.

If one of the disk's sectors gets damaged, the computer might get so confused that it handles the entire file incorrectly, and so the entire file becomes unusable.

To minimize such damage, avoid creating large files; create many small files instead. For example, if you're writing a book and want to store the book on your disk, do not make the entire book be a single file; instead, split the book into chapters, and make each chapter a separate file. That way, a damaged sector will hurt at most one chapter, and can't hurt the entire book.

Don't bother trying to clean the heads of your floppy drive. The heads don't collect much dirt anyway, since the floppy disk's jacket has a cloth liner that traps most dirt. If your disk ever starts to act unreliable, clean the heads if you wish, but the culprit is more likely a misaligned head, a brownout, overheating, defective software, or a mistyped command.

Hard disks

Hard disks are better than floppy disks in three ways....
Hard disks are sturdier than floppies.
Hard disks are hard and firm; they don't flop or jiggle.
They're more reliable than floppies.

Hard drives hold more information than floppy drives.
The typical floppy drive holds 360K, 720K, 1.2M, or 1.44M.
The typical hard drive holds 250M, 340M, or 420M.

Hard drives work faster than floppies.
The typical floppy disk rotates between 5 and 10 times per second.
The typical hard disk rotates between 60 and 120 times per second.

Hard drives are more expensive than floppy drives. The typical floppy drive costs about $50; the typical hard drive costs about $300.

Unfortunately, the typical hard disk can't be removed from its drive: the hard disk is non-removable, stuck inside its drive permanently. (Hard disks that are removable are rare.)

Since the typical hard disk is stuck forever inside its drive, in one fixed place, it's called a fixed disk.

Though the typical floppy-disk drive holds just one disk at a time, the typical hard-disk drive holds a whole stack of disks and handles all the stack's disks simultaneously, by using many arms and read-write heads. For example, the typical 420M hard drive holds a non-removable stack of disks, and the entire stack totals 420M. Each disk in the stack is called a platter.

If your hard drive is the rare kind that holds a removable stack of disks, the stack comes in a cartridge or pack that you can remove from the hard drive.

Back in 1977, the typical hard disk had a 14-inch diameter and was removable. The hard-disk drive was a big cabinet, the size of a top-loading washing machine; it cost about $30,000 and held 100M. It required a minicomputer or mainframe.

Hard disks, drives, and prices have all shrunk since then! Now the typical hard disk has a diameter of just 31/2 inches. The typical hard drive is just 1 inch tall, costs $300, and holds 3100M. It fits in a desktop microcomputer.

Some notebook computers use tiny hard disks whose diameter is just 21/2 inches.

IBM drive letters

The typical IBM-compatible computer has both a floppy drive and a hard drive. The floppy drive is called drive A; the hard drive is called drive C.

If the computer has two floppy drives, the main floppy drive is called drive A, and the other floppy drive is called drive B. If the computer has two hard drives, the main hard drive is called drive C, and the other hard drive is called drive D.

Copy from floppy to hard & back

When you buy a program, it usually comes on a floppy disk. To use the program, put that floppy disk into the floppy drive, then copy the program from the floppy disk to the hard disk. (To copy the program onto an IBM-compatible hard disk, type the word "copy" or "install" or "setup". To find out which of those three words to type and when, follow the instructions in the manual that came with the program.)

Then use just the copy on the hard disk (which is sturdier, holds more info, and works faster than the floppy disk).

Like floppy disks, hard disks are coated with magnetized iron. Floppy disks and hard disks are both called magnetic disks. Like floppy disks, hard disks are in constant danger of losing their magnetic signals - and your data!

Protect yourself! Every day, take any new info that's on your hard disk and copy it onto a pile of floppy disks, so that those floppy disks contain a backup copy of what was new on your hard disk.

To avoid giant disasters, avoid creating giant files. If you're writing a book and want to store it on your hard disk, split the book into chapters, and make each chapter a separate file, so that if you accidentally say "delete" you'll lose just one chapter instead of your entire masterpiece.

How the head works

In a floppy drive, the read-write head (the "needle") touches the spinning floppy disk. But in a hard drive, the read-write head does not touch the spinning hard disk; instead, it hovers over the disk.

The distance from the read-write head to the hard disk is a tiny fraction of an inch, and small enough so that the read-write head can detect the disk's magnetism and alter it.

Since the head doesn't actually touch the disk, there isn't any friction, and so the head and the disk don't suffer from any wear-and-tear. That's why a hard-disk system lasts longer than a floppy-disk system and is more reliable.

Winchester drives
In all modern hard drives, the head acts as a miniature airplane: it flies above the disk. It flies at a very low altitude: a tiny fraction of an inch. The only thing keeping the head off the rotating disk is a tiny cushion of air - a breeze caused by the disk's motion.

When you unplug the drive, the disk stops rotating, so the breeze stops, and the head comes to rest on a landing strip, which is like a miniature airport.

Such a drive is called a flying-head drive. It's also called a Winchester drive, because "Winchester" was IBM's secret code-name for that technology when IBM was inventing it.

The head flies at an altitude that's extremely low - about a ten-thousandth of an inch! That's even smaller than the width of a particle of dust or cigarette smoke! So if any dust or smoke lands onto the disk, the head will smash against it, and you'll have a major disaster.

To prevent such a disaster, the entire Winchester drive is sealed, to prevent any dust or smoke from entering the drive and getting onto the disk. Since the drive is sealed, you can't remove the disks (unless you buy an extremely expensive Winchester drive that has a flexible seal).


Here's how the computer retrieves data from the drive.
First, the drive's head moves to the correct track.
The time that the head spends moving is called the seek time. Since that time depends on how far the head is from the correct track, it depends on where the correct track is and where the head is moving from.

According to calculus, on the average the head must move across a third of the tracks to reach the correct track. That's why the time to traverse a third of the tracks is called the average seek time.

A millisecond (ms) is a thousandth of a second. In a typical hard drive, the average seek time is 12 milliseconds. (In the fastest hard drives, the average seek time is 8 milliseconds; in the slowest hard drives made today, the average seek time is 14 milliseconds; in older hard drives that are no longer made, the average seek time was 28 milliseconds.)

After the head reaches the correct track, it must wait for the drive to rotate, until the correct sector reaches the head.

That rotation time is called the latency. On the average, the head must wait for half a revolution; so the average latency time is a half-revolution. The typical hard drive rotates 90 times per second, so a half-revolution takes half of a 90th of a second, so it's a 180th of a second, so it's about .006 seconds, which is 6 milliseconds.

If you add the average seek time to the average latency time, you get the total average access time. So for a typical hard drive, the average access time = 12 milliseconds seek + 6 milliseconds latency = 18 milliseconds.

During the last few years, hard drive manufacturers have become dishonest: they say the "average access time" is 12 milliseconds, when they should actually say the "average seek time" is 12 milliseconds.

After the head finally reaches the correct sector, you must wait for the head to read the data. If the data consumes several sectors, you must wait for the head to read all those sectors.


Most hard drives for microcomputers are manufactured by three American companies: Seagate Technology (ST), Quantum, and Western Digital.

Seagate was the first of those companies to make hard drives for microcomputers, and it set the standard that the other companies had to follow. New Seagate drives work fine, though Seagate's older models were often noisy and unreliable.

Quantum became famous by manufacturing the hard drives that Apple buys to put in Mac computers. Quantum also builds drives for IBM PC clones. Quantum drives are excellent.

Western Digital has invented hard drives that cost less. They're popular in cheap clones and discount computer stores.

Conner was the first company to invent hard drives tiny enough to fit in a laptop or notebook computer. Seagate ignored the laptop/notebook market too long, and Conner's popularity zoomed up fast. Conner became the fastest-growing company in the history of American industry! But then Conner's competitors caught up and Conner's popularity sunk back down. In Febuary 1996, Seagate bought Conner.

Other popular manufacturers of hard drives are America's Maxtor & Micropolis, Japan's NEC & Fujitsu, and Korea's Samsung. When buying a hard drive, you might also need to buy a hard-drive controller.

How many sectors?

Back in the 1980's, the typical hard-drive controller for IBM-compatible computers put 17 sectors on each track. That scheme was called the Seagate Technology 506 with Modified Frequency Modulation (ST506 MFM).

An improved scheme, which squeezed 26 sectors onto each track, was called the ST506 with Run Length Limited (ST506 RLL). A further improvement, which squeezed 34 sectors onto each track, was called the Enhanced Small Device Interface (ESDI).

Squeezing extra sectors onto each track increases the drive's capacity (total number of megabytes) and also the transfer rate (the number of sectors that the head reads per rotation or per second).

All those schemes - MFM, RLL, and ESDI - have become obsolete.

Now the most popular scheme is called Integrated Drive Electronics (IDE). Like ESDI, it squeezes 34 sectors onto each track; but it uses special tricks to transfer data faster.

The original version of IDE was limited to small drives: up to 528M. A new, improved version, called Enhanced IDE (EIDE), goes faster and can also handle bigger drives. Enhanced IDE was invented by Western Digital and is also used by Conner and Maxtor. Seagate invented a competing standard called Fast ATA-2, which is used by Seagate, Quantum, Fujitsu, and Samsung. You can buy Enhanced IDE and Fast ATA-2 drives that hold up to 3840M or even 6510M!

A totally different fast scheme is the Small Computer System Interface (or SCSI, which is pronounced "scuzzy"). It's used on most Mac hard drives and the biggest IBM-compatible hard drives. The newest, fastest version of SCSI is called Ultra Wide SCSI. You can buy Ultra Wide SCSI drives that hold up to 9100M!

Discounts on drives

You can buy hard drives cheaply from these discount dealers:
Dirt Cheap Drives
usually lowest prices; ships just to USA; open weekdays 8AM-8PM, Saturday 9AM-3PM, Central Time
3716 Timber Drive
Dickinson TX 77539
phone 800-473-0960 or 281-534-4140

low prices; ships to USA & Canada; open weekdays 8AM-8PM, Saturday 9AM-3PM, Central Time
2201 Pine Drive
Dickinson TX 77539
phone 800-786-1185 or 281-534-3919

Insight (and its division called Hard Drives International)
slightly higher prices; ships to all countries; open always (every day & night, round-the-clock, 24 hours)
1912 W. Fourth St.
Tempe AZ 85281 phone 800-INSIGHT or 602-902-1176

IBM-compatible drives
Modern, popular IBM-compatible hard drives are IDE, hold 3 or 4 gigabytes, and cost about $100 per gigabyte. For example, a 3-gigabyte drive costs about $300; a 4-gigabyte drive costs about $400.

Besides buying the hard drive, you must also buy a card to put in the computer's slot (unless your computer contains such a card already). For example, here are the prices charged by Dirt Cheap Drives:

Capacity Seek Rotation Cache Type Brand Model Price
1700M 10 ms 5400 rpm 128K Fast ATA-2 Fujitsu M 1623T $192+$33
2500M 10 ms 5400 rpm 128K Fast ATA-2 Fujitsu M 1638T $225+$33
3166M 12 ms 5200 rpm 128K Enhanced IDE Western Digital AC3 3100 $292+$33
3228M 10 ms 5400 rpm 128K Fast ATA-2 Quantum FB3 3240A $309+$33
4310M 10 ms 5400 rpm 128K Fast ATA-2 Quantum FB3 4320A $375+$33
5120M 10 ms 5400 rpm 256K Enhanced IDE Maxtor M8 5120A $449+$33
6448M 10 ms 5400 rpm 128K Fast ATA-2 Quantum FB3 6480A $539+$33
9091M 9 ms 7200 rpm 512K Ultra Wide SCSI Fujitsu MC 3391W $1455+$199
23300M 14 ms 5400 rpm 512K Ultra Wide SCSI Seagate ST4 23451W $3899+$199
At the end of the model number, an A or T means AT-bus IDE (or Enhanced IDE or Fast ATA-2); a W means Wide SCSI.

A cable runs from the drive to a card, which fits into an IBM AT slot. For a drive that's Enhanced IDE (or Fast ATA-2), the best kind of card is an IDE accelerator, which costs $33 ($38 minus $5 discount). For Ultra Wide SCSI drives, get an Ultra Wide SCSI controller card instead, which costs $199. In the table above, the "+$33" or "+$199" is for those cards.

For Maxtor, Seagate, and Quantum drives, the main part of the model number is the approximate capacity. For Western Digital drives, the main part of the model number is the capacity rounded down to the nearest 100.

The drive's cache (or buffer) contains RAM chips that hold copies of the sectors you used recently - so that if you want to look at those sectors again, you can read from the RAM chips (which are fast) instead of waiting for the disk to spin (which is slow).

Those were the prices in April 1997. By the time you read this book, prices might be even lower! Prices continually drop.

Mac drives
The price of a Mac hard drive depends on whether the drive is internal (fits inside the Mac) or external (comes in a separate box that you put next to the Mac). Internal drives are cheaper; but if your Mac is small or filled up, you must buy an external drive instead.

For Mac drives, here are Megahaus prices:

Capacity Seek Rotation Cache Brand Model Internal External
1280M 12 ms 4500 rpm 128K Quantum TM3 1280S $229 $289
2110M 11 ms 4500 rpm 128K Quantum TM3 2110S $300 $360
3254M 11 ms 4500 rpm 128K Quantum TM3 3200S $345 $405
4310M 10 ms 5400 rpm 128K Quantum FB3 4320S $459 $519
6448M 10 ms 5400 rpm 128K Quantum FB3 6480S $574 $634
9100M 8 ms 7200 rpm 512K Quantum XP3 9100W $1429 $1549
23200M 13 ms 5400 rpm 512K Seagate ST4 23451W $3899 $4099
Buy a big drive
Back in the 1980's, a 40-megabyte drive was considered "big", and it was the most popular size to buy.

In the 1990's, a 40-megabyte drive is considered "too small". Even a 540-megabyte drive is considered "too small".

Here's why:
To use modern programs on an IBM-compatible computer, you must make sure the hard disk contains some fundamental software, called MS-DOS and Windows. The newest version of MS-DOS (version 6.22) consumes over 5 megabytes. Windows 3.11 consumes over 10 megabytes. That makes 15 megabytes altogether! (Windows 95 consumes even more: just the "important" parts of Windows 95 consume over 35 megabytes!)

The typical Windows word-processing program (such as such Word Perfect 6.1 or Microsoft Word 6) consumes about 25 megabytes. So altogether, for DOS plus Windows plus a word-processing program, you've consumed over 40 megabytes already!

You'll need additional megabytes for additional business programs (about 10 megabytes per program), plus additional megabytes to hold what you type. After buying the computer, you'll probably spend the next several years accumulating many programs (a few each year). After a year or two, you'll accumulate over 540 megabytes, and you'll wish you'd bought a bigger drive instead.

Buy at least a 1080-megabyte drive. A 1080-megabyte drive costs just slightly more than a 540M drive and will last you for many years. You're buying peace of mind! It's much cheaper to buy a 1080-megabyte drive now than to buy a 540-megabyte drive now and another 540-megabyte drive later. Another reason for buying a 1080-megabyte drive is that it will act faster than a 540-megabyte drive.

For example, suppose you want to store 540 megabytes of information, and you're debating whether to buy a 540-megabyte drive or a 1080-megabyte drive. Suppose each drive is advertised as having a 10-millisecond seek time. The 1080-megabyte drive will nevertheless act faster.

Here's why....
Suppose you buy the 1080-megabyte drive and use just the first 540 megabytes of it. Since you're using just the first half of the drive, the head needs to move just half as far as usual; so over the 540-megabyte part that you're using, the effective average seek time is just half as much as usual: it's 5 milliseconds!


Instead of buying a program on a floppy disk, you can buy a program on the same kind of compact disk (CD) that holds music. Since the CD cannot be erased, it's called a CD read-only memory (CD-ROM).

To make your computer read the CD-ROM disk, put the disk into a CD-ROM drive, which is a souped-up version of the kind of CD player that plays music.

Like an ordinary CD player, a CD-ROM drive uses just optics. No magnetism is involved. The drive just shines a laser beam at the shiny disk and notices, from the reflection, which indentations (pits) are on the disk; the pattern of pits is a code that represents the data. So a CD-ROM drive is an example of an optical disk drive.

To put the disk into the drive, press a button on the drive. That makes the drive stick its tongue out at you! The tongue is called a tray. Put the disk onto the tray, so that the disk's label is face-up. (If the drive is old-fashioned, you must put the disk into a caddy first; but the most modern drives are caddyless.) Then push the tray back into the drive. Finally, use the keyboard or mouse to give a command that makes the computer taste what you've put on its tongue.

IBM drive letters

In the most modern kind of IBM-compatible computer, drive A is a 31/2-inch floppy drive (1.44M), drive B is a 51/4-inch floppy drive (1.2M), drive C is a hard drive (holding about 420M), and drive D is a CD-ROM drive.

But if your computer has two hard drives, here's what happens: the first hard drive is C, the second hard drive is D, and the CD-ROM drive is E.


CD-ROM disks come in two sizes:
The standard size has a diameter of 12 centimeters (which is about 5 inches) and holds 540 megabytes.
The miniature size has a diameter of 8 centimeters (which is about 3 inches) and holds 180 megabytes.

Your CD-ROM drive can handle both sizes of CD-ROM disks. Each CD-ROM disk is single-sided: all the data is on the disk's bottom side - the side that doesn't have a label.

Yes, a standard-size CD-ROM disk holds 540 megabytes, which is a lot!

It's more than the typical hard drive.
It's 375 times as much as a high-density 1.44M floppy.
It's 1500 times as much as a 360K floppy.

Because a CD-ROM disk holds so much, a single CD-ROM can hold a whole library (including encyclopedias, dictionaries, other reference materials, famous novels, programs, artwork, music, and videos). It's the ideal way to distribute massive quantities of information! Moreover, a CD-ROM disk costs just $1.50 to manufacture (once you've bought the appropriate CD-ROM-making equipment, which costs several thousand dollars).


When buying a CD-ROM drive, the most important factor to consider is the drive's speed.

Transfer rate
The speed at which the drive spins is called the transfer rate. The higher, the better!

On the first CD-ROM drives that were invented, the transfer rate was the same speed as music CD's: 150 kilobytes per second.

Then came drives that could spin twice as fast: 300 kilobytes/second. That's called double speed or 2X.

Then came drives that could spin thrice as fast: 450 kilobytes/second. That's called triple speed or 3X.

Then came drives that could spin four times as fast: 600 kilobytes/second. That's called quad speed or 4X.

Then came 41/2X drives, then 6X, then 8X, then 10X, then 12X. Most drives that are sold today are 8X or 12K. The 8X drives transfer 1200 kilobytes/second; the 12X drives transfer 1800 kilobytes/second.

16X drives have been invented, but they're too expensive to be popular yet.

Seek time
The average time it takes for the head to move to the correct track is called the average seek time.

The lower the average seek time, the better!

Under 200 milliseconds is good.
200-300 milliseconds is acceptable.
Over 300 milliseconds is terrible.

Buying a drive

Single-speed, double-speed, and triple-speed drives aren't sold anymore.

Here are the cheapest good drives:

Transfer rate Seek Buffer Brand Model Price
8X 140 ms 256K NEC CDR 1400 $109
12X 150 ms 256K Cyber Cyber 12X $115
12X 125 ms 128K Acer AC 612A $120
12X 115 ms 256K Toshiba XMS 702B $141
12X/16X 100 ms 128K NEC CDR 1600 $149

Each of those drives is IDE. The 12X/16X drive handles the inner tracks at 12X and the outer tracks at 16X. You can get those prices from these discount dealers in Dickinson, Texas:
Dirt Cheap Drives
3716 Timber Drive
Dickinson TX 77539
phone 800-473-0960 or 281-534-4140

2201 Pine Drive
Dickinson TX 77539
phone 800-786-1185 or 281-534-3919

External drives
Those prices are for internal drives, which fit inside the computer's system unit. If your system unit is filled up and doesn't have any room left to insert an internal drive, you must buy an external drive instead, which sits outside the system unit and costs about $30 more.

Multimedia kits
If you buy a CD-ROM drive, you'll also want a sound card, a pair of stereo speakers, and a few sample CD-ROM disks (so you can admire all that equipment you bought). That combo - a CD-ROM drive, sound card, pair of speakers, and sample CD-ROM disks - is called a multimedia kit.

The most popular fast multimedia kit is the Sound Blaster Discovery CD 12X. It includes a 12X CD-ROM drive, a Sound Blaster 32-bit wave-table sound card, a pair of stereo speakers, and 12 programs. Get it for just $335 from discount dealers such as Harmony Computers (in New York at 800-441-1144 or 718-692-3232). Harmony also sells an older version, the Sound Blaster Discovery CD 8X, for just $245.

Tough installation

Warning: CD-ROM drives (and multimedia kits) are extremely difficult to install correctly because they tend to conflict with other hardware and software. Even if you follow the instructions in the setup manual, the stuff typically doesn't work.

If you try to get help by phoning the company that made the multimedia kit (such as Creative Labs), you almost always get a busy signal. Even if you finally get through to a technician, the technician can't help you much, since the technician doesn't know enough details about the other hardware and software your computer contains.

Most consumers give up in disgust, return the stuff to the store, and pay the store about $70 to do the installation. Some stores offer "free" installation - but do so by advertising a higher price for the CD-ROM drive.

When you buy a new computer, you can ask the salesperson to include a multimedia kit and install it. A computer that includes an installed multimedia kit is called a multimedia computer system.


A CD-ROM disk's main enemy is dirt.

When you buy a CD-ROM disk, it comes in a clear square box, called the jewel box. To use the CD-ROM disk, remove it from the jewel box and put the disk into the drive. When you finish using the disk, put it back into the jewel box, which keeps the dust off the disk.

When putting the CD-ROM disk into or out of a drive, don't put your fingers on the disk's surface: instead, hold the disk by its edge, so your greasy fingerprints don't get on the disk's surface.

Once a month, gently wipe any dust off the CD-ROM disk's bottom surface (where the data is). While wiping, be gentle and don't get your greasy fingerprints on the disk. Start in the middle and wipe toward the outer edge.

For example, my assistant and I were getting lots of error messages when using a sample CD-ROM disk we bought from Microsoft. I was going to phone Microsoft to complain, but my assistant asked, "What about dust?" I flipped the CD-ROM disk over and sure enough, a big ball of dust was on the disk's bottom side, where the data is recorded. I wiped it off. That CD-ROM disk has worked perfectly ever since.

I was so embarrassed! If my assistant hadn't reminded me to wipe the dust off, I'd have wasted hours of Microsoft's time hunting uselessly for a high-tech reason my CD-ROM disk wasn't working.

Other dangers

Don't put any fluids on the disk. The fluids that clean phonograph records will wreck CD-ROM disks.

If you want to write on the disk, use a felt-tipped pen (not a ballpoint or pencil). Don't stick any labels on the disk.

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