andreas.com FAQ: DisksFAQ: Disks, Disk Drives, and Hard Disks
(This FAQ was written back in the mid-90s, when we still used diskettes. It’s now obsolete, but some of it may be interesting. There’s a bit about hard disks at the end.)
An Introduction to Media Storage
You’re writing a letter on your computer. So that you can turn off the computer, you need to save the letter so that you can continue writing tomorrow. The letter has to be stored somewhere. It will be saved on the diskettes or the hard disk which both do the same thing. They hold information. Since they can hold information, even when the power is off, we call it non-volatile media, which means that they don’t evaporate when you turn off the power.
You open the letter and it appears on your screen. That information is also held in a working space. Every time you press a key and add a new character or delete one, the letter is updated. The working storage space must be very fast so that you can scroll around, delete sentences, and move paragraphs. The letter is thus held in RAM memory chips. You can work with your file without waiting for it to be updated. The down side of this is that the RAM chips are volatile media ; if you lose power accidentally, your work evaporates. When you turn off the computer, the letter will evaporate from RAM.
When you finish your letter, you save it by writing it back onto the non-volatile media, which means that the letter will remain there, even when you turn off the computer.
Non-volatile media can be anything that can record and read information. This includes magnetic recording systems such as tape, disks, and hard disks. There are also laser-based reading systems such as CD-ROMs and holographic cubes. Years ago, paper punch cards were used. Electronic systems, such as flash RAM, are battery powered cards of RAM chips. The battery prevents the files from disappearing. These are used in PDAs and digital cameras. There are also mechanical storage systems: students have built computers out of Tinker Toys. Before electronic pocket calculators, there were mechanical calculators which could perform simple arithmetic.
Disks vs. Discs
Disks are the same thing as cassette tapes. Disks have a coating of magnetic ferrous oxide, or iron rust, on both sides of a sheet of mylar (plastic.) The thin plastic brown disk inside the diskette is called the cookie. Marketing people don’t use diskette. They think it sounds wimpy. The hard disk is spelled disk, but the metal disk inside is spelled disc. A computer CD is the same type of thing as a music CD. A CD is spelled disc. It’s a trademark thing.
How Does a Disk Really Work?
A disk and a disk drive are somewhat like a record and a record player. The disk holds the files, just as a record holds the music. The disks, however, don’t have grooves. The disk has a smooth surface which is magnetized. When you format a disk, it’s divided into circular tracks, each of which is divided into ten sectors or so. This is just like the painted lines on a parking lot. The lines are magnetic lines. Of course, the size and numbers of the sectors can be adjusted to hold larger or smaller files.
Take an old, used disk. Slide back the metal shutter and hold the disk at an angle to a bright light. You’ll see very small shiny concentric lines. These are the tracks.
The disk drive has an arm, just like a record player’s arm. Instead of a needle, however, the tip of the head is a small electromagnet. When the drive is writing to the disk, the head magnetizes the disk surface in a sequence of positive or negative magnetic fields. Each field is a bit. For example, the letter B, which may be the eight-bit set, written as “10011011,” will be a series of magnetic fields: positive, negative, negative, positive, positive, negative, positive, positive.
When the drive is reading a disk, the head passes over the disk and reports the sequence of the positive or negative magnetic fields.
The diskette spins about 300 times per minute. Hard disks spin about 3,000 to 6,000 times per minute.
Different Kinds of Disks
There are different types of disks:
- single-sided (SS) vs. double-sided (DS)
- Double density (DD) vs. high density (HD)
- 5¼” vs. 3½”
Let’s look at each of these types.
single-sided (SS) vs. double-sided (DS)
All disks are made with the coating on both sides. This means that all disks are really double-sided. The manufacturer then tests the disks. A single-sided disk is just tested on one side (actually, the back side). A double-sided disk is tested on both sides.
Single-sided disks are cheaper because it’s faster to test only one side. You can use a single-sided disk as a double-sided disk. If it can be formatted, then it’ll work. However, to be safe, you shouldn’t do this. If it’s a low quality disk, then the oxide coating can’t keep a magnetic fields
Double Density (DD) vs. High Density (HD)
The density of a disk has to do with the amount of iron on its surface. Single density disks are obsolete. There are double density (DD) and high density (HD) disks.
- A double-sided, double density (DS, DD) disk holds 720 KB (which is 0.7 MB) or roughly 350 pages of text.
- A double-sided, high density disk (DS, HD) holds 1,440 KB (which is 1.44 MB) or roughly 700 pages of text. Of course, high density disks are better in that they hold twice as much.
Converting DD Disks into HD Disks
Some companies sell a hole puncher which lets you punch a hole in the corner of a double density disk so that it becomes a high density disk.
You don’t have to buy one of these hole punchers. You can do this with a pocket knife. The HD disk drive only checks the hole on the backside of the disk. Reverse a second disk and put it over the back of your disk. Use a pencil to mark the position of the square. Use a knife or slow drill to cut into the back side of the disk and create a dimple. You don’t need to penetrate to the front side. Try the disk. If you can’t format it as a 1.44 MB disk, just cut a little bit more.
Disk manufacturers don’t recommend this. The disk’s density can’t hold the high density of data. A double density disk (DD) has 6816 bits per inch (bpi) on a track. A high density has 13,632 bpi. You may lose data.
What Kind of Disk Is This?
- If you can flex the disk by waving it, then it’s a 5¼” floppy disk. Oddly, the density of 5¼” disks isn’t marked on the cover. If the disk has a reinforcement ring in the hub, then it’s a double density (DD) disk. If it doesn’t have a reinforcement ring, then it’s a high density (HD) disk.
- If they are in hard plastic cases, then they are 3½” disks. If the 3½” disk has only one hole in the corner, then it’s a double density (DD) disk. If the 3½” disk has two holes in the corners, then it’s a high density (HD) disk. HD disks are often stamped HD.
The Write Protect Tab
In the corner of your disk is either a notch (5¼” disks) or a sliding tab (3½” disks). A disk drive uses either a spring or an infrared light to see if the hole is open. If it’s open, then the drive won’t write to the disk. When the disk is write-protected, you can’t delete files, accidentally reformat it, catch a virus, etc.
- To protect a 5¼” disk, use a label (or a piece of non-transparent tape) so that the notch is closed. Nothing can be changed now.
- To protect a 3½” disk, slide the tab so that the hole is open. Nothing can be changed now.
Cheap vs. Expensive Disks
The other difference between disks is that some are cheap and others are expensive. Which ones should you buy?
The difference is in the quality of the iron oxide coating on the disk. Cheap disks, which don’t have the manufacturer’s name stamped on the metal shutter, are called no names. No names are either cheap disks or disks which a maker of good disks has dumped onto the market to undercut the no name manufacturers. Generally, cheap disks don’t last very long, especially if you buy them in a supermarket or warehouse type of store. You’ll find that often 20-30% of no name disks can’t be formatted or won’t hold a file for very long. You’ll end up replacing them or losing files.
A reputable manufacturer sets a higher standard. It’s cheaper to buy expensive disks which last longer. Diskette Gazette, the journal of the disk duplicating industry, rates as excellent the 3½” disks by Verbatim, Sony, Itoh, TDK, JVC, Maxell, and KAO.
Nearly all 5¼” disks are excellent, since all manufacturers have learned how to make perfect disks.
What Shall I Do with a Bad Disk?
Collect them until you have a box of bad disks and then return them to the manufacturer. They usually will replace the disks for free.
How to Format Disks under DOS
Put the disk in the drive and, at the DOS prompt, type DIR. If the disk is formatted, it’ll report the contents. If the disk is unformatted, then a message will appear that the disk is unformatted.
To format the disk, just type at the DOS prompt FORMAT A: (or B: if in drive B) and press Enter.
When it finishes, you can choose to add a volume label. This can be a name for the disk, or you can just press Enter again and leave it blank.
Finally, DOS will ask you if you want to format another disk. Press Y(es) or N(o). That’s all.
How to Format a Double Density Disk under DOS
If you want to use a high density drive to make a disk which will be used on a double density drive, then you have to tell your computer to use a double density format. If the disk is a double density 5¼” disk, at the DOS prompt, type FORMAT A: /F:360. If the disk is a double density 3½” disk, at the DOS prompt, type FORMAT A: /F:720.
How to Format Disks under Windows
Put the disk in the drive and, from the File Manager, click on the A or B drive icon. If the disk is formatted, it’ll report on the contents. If the disk is unformatted, then a message will appear that the disk is unformatted. Follow the instruction to format it.
Does It Hurt My Disk To Reformat It Often?
The drive head is in direct contact with the surface of the diskette. The more it’s used, the sooner the disk wears out.
Whoops! How to Unformat
If you accidentally format a disk, and you realize that you had files on it, you can unformat it if you have DOS 6.0 or higher. Type UNFORMAT A: (or B:).
How Much Room Is on My Disk?
In DOS, type the command CHKDSK A: or CHKDSK B: and press Enter. This will report on the size of the diskette and how much space is available.
In Windows, open the File Manager. Select View | Tree and Directory so that a check mark appears. Select Options | Status Bar so that a check mark appears. Click on the drive icon at the top of the root (A:, B:, or C:). At the bottom of the File Manager, it tells you how much space is available and how large the disk is.
How Long May I Store a Disk?
If it’s a name-brand disk and you store it in a dry place at room temperature, it’ll last about ten years. After that, you should transfer the data to a new disk.
Backup tapes last about 40 years.
How to Recover Lost Files
If you accidentally delete a file, you can usually find it in the wastebasket. If it’s not there, it may be possible to recover the file. This can become rather complex. Don’t touch your computer. Don’t save any files or turn it off. Contact your computer consultant for help.
Taking Care of Your Disks
- Keep your disks in a disk box. This can be just a shoe box. It must have a cover to keep dust out.
- Keep your disks upright to keep the dust out.
- Don’t keep your disks in a desk drawer. Put them on a shelf. If you spill coffee on the desk, it’ll pour right into an opened drawer (yes, this happens!).
- Keep the 5¼” disks in their sleeves to protect the disk surface. 3½” disks don’t need to be kept in the plastic sleeves.
- Disks shouldn’t go below 50°F (10°C) or higher than 122°F (50°C). If the disk has been frozen or baked, throw it away.
- Keep your disks away from magnetic fields, such as telephones, screens, and TVs (the field is much stronger at the sides of the cabinet). Your disks should also avoid the little magnets in the desktop paper clip holder. Scissors and paper clips are often slightly magnetized as well.
- Don’t smoke near your diskettes.
- Don’t hold your disks together with a rubber band.
- Don’t put disks in your shirt pocket. It’ll get filled with lint.
- Don’t use a pencil or ball-point pen to write on a disk label. You’ll press the dust sleeve inside of the disk case against the disk. The dust will be caked onto the surface of the disk. Much worse, however, is that you’ll indent the surface of the disk. Use a felt tip pen and write lightly.
- Keep backup disks on the other side of town in case of fire or burglary.
The Most Common Disk Problems
- Writing on the disks
- Cigarette smoke
A great way to protect disks from dust, water, etc., is to put them into a Ziploc baggy.
The Disk Fell into the Pool. Is It Okay?
Yes. Just let it dry before you put it into the drive. You may want to copy the data onto a new disk and throw away the old disk.
Special Disks for Dirty Work
If your work is especially dusty or greasy, then you can get Teflon-coated disks from Verbatim. Where a name brand disk may lose 99% of the data, a Verbatim DataLife disk will only lose 5%.
Sending Disks by Mail
Just put a disk into an envelope and send it off. I have sent tens of thousands of disks in ordinary envelopes with only a cover letter wrapped around them. Computer magazines just tape them to the front of the magazine and send them out. They often send out ½ million diskettes at a time.
It may protect the disk a bit more, however, if you add a layer of cardboard. You can also buy disk envelopes, which are stiff cardboard envelopes, but these are not necessary.
Airport X Rays, Cosmic Rays, and Other Fairy Tales
Many people write that airport X rays are okay, but that the magnetic fields of the transformers under the conveyor belts may cause trouble. It’s not true.
It is perfectly safe to send your disks through the airport X ray camera. Tens of millions of disks in books, program packages, and magazine covers are sent through airport X ray bomb detectors and post office conveyor belts every month without any problem.
You’ll also hear of stray cosmic rays which can zap a bit on a disk. There are better things to worry about.
Write on the labels with a felt-tipped pen or lightly with a pencil. Don’t write with a ball-point pen. That’ll crease the surface of the disk and press dust onto it.
Don’t use a knife to scrape off the label. You’ll press dust onto the surface of the disk. Don’t remove disk labels with nail polish remover or acetone. That’ll remove the plastic as well.
Look around and you can find removable disk labels. If you get tired of peeling off labels and hate the whole mess of non-removable labels, there are small plastic pockets for disk labels. You can slide the text of the month into the pocket. You can print out a list of what is on the disk and slip it into the pocket. Call Labelit at 800-627-7752.
My Disks Are Curly!
A customer complained that the office disks didn’t fit in the drive because the disks were curly. A technician was sent to the office. It turned out that they were putting the disks into the typewriter to type the labels.
The Disk Drives
A disk drive is somewhat like a record player. An arm with a magnetic head at the tip swings over the disk. To save a document to the diskette, the magnetic tip is switched off and on at a high speed which leaves a track of magnetic signals, either positive or negative.
To read a document, the same head passes over the disk and reports on the pattern of positive and negative spots.
The diskette itself is shoved in and out of the disk drive. As the disk slides in, the drive head is raised and the metal shutter on the 3½” disk slides back. If the drive is double-sided, there is a drive head on both sides of the disk. When the disk sits securely in place, the drive head presses down against the disk surface. This friction means that the disk will wear out. It also means that the drive head may become dirty.
A: and B: Disk Drives
Older computers have two disk drives. The opening of one is about six inches wide and has a latch which you must close whenever you put a disk into the drive. This is the 5¼” drive. The opening of the other drive is about three inches wide and has a button to eject the diskette. This is the 3½” drive.
From the DOS prompt, type A: and press E. Watch the lights on the disk drives. The one which lights up is your A: drive.
Try the same thing with your other drive. Type B: and press Enter. The light should light up on the B: drive.
It’s very helpful to put a small label on your drives and write A: and B: so you’ll know which is which.
The Boot Drive
One of the drives is the boot drive or the start up drive. If you put a disk in the drive and then turn on the computer, the computer will use the information from that disk to start itself.
To find out which is the boot drive, take all disks out of the drives and turn on the computer. The light on the boot drive will briefly light. You can also note this on a label and put it over the boot drive.
Double Density Drives
Double density drives can only use double density (DD) disks. A double density drive can also read high density (HD) disks which have been formatted to double density format (720 KB). They can’t read a high density disk which has been formatted to 1.44 MB. Luckily, double density drives aren’t sold any longer.
High Density Drives
Your high density drive can use both high density (HD) and double density (DD) disks.
Removing Disks While the Drive Is Running
The head is automatically raised when the disk is sliding in or out. It won’t damage the disk or drive to take disks out while the drive is running.
However, if you’re reading or writing to the disk and you insert or remove the disk, the computer may not read or write the information properly. So it won’t damage the disk, but information may be lost.
This is good to know if you begin to erase or reformat and you realize that the wrong disk is in the drive. It takes a moment for the drive to begin to spin and another moment for the head to find its position, which gives you enough time to pop the disk out of the drive.
Remember to Remove the Disk Before Inserting a New Disk
Be careful not to shove a second disk into a 5¼” drive without removing the disk first. It’s easy to do this.
It’s also easy to shove a 5¼” disk between the drives. If it gets in there, open the cabinet to get at the diskette.
Why Does the Disk Work on Her Computer but Not on Mine?
If your computer is several years old, you may notice that your computer only likes its own disks. It can only use the disks which have been formatted on it. It can’t use disks from other computers. You’ll complain to manufacturers that they sent you bad disks. The disks from your computer can’t be used on other computers either. People will complain that you sent them a bad disk.
This is a head alignment problem. It’s a common problem with old disk drives. Over the years, the drive heads may drift out of alignment. Instead of formatting tracks at a standard distance, the head may be slightly to the side. This means that disks from that computer can’t be read by other computers, nor can the computer read other disks.
To be sure that this is the problem, format a new disk on your computer. Copy a large file onto it. Put the disk into a new computer and copy the file onto the new computer. If the new computer can’t copy your file, try it on yet a second new computer. If that doesn’t work either, then your head is out of alignment. It may help to clean the head.
There’s little to do about this. The head can be realigned, but that will cost more than a new drive. Usually you just replace the drive.
There will be a problem, however. If you have formatted disks with your misaligned drive, then you won’t be able to read those disks with the new drive. You’ll have to first copy those disks onto your hard disk, change the drives, reformat the disks, and then copy the files back onto the disks.
You can also try a box of Verbatim disks. These are very high quality disks with more density than other disks. The data can often be read, even when the tracks are slightly misaligned.
Should I Clean My Drive Head?
In a disk drive, the head is pressed against the disk, just like in a cassette tape player. Some of the diskette’s iron oxide surface coating will build up on the disk drive’s head. The layer may make it difficult to read or write to a disk.
Computer stores sell little kits to clean your drives. These should include a special diskette which is actually a scrubber. The cleaning fluid, which dissolves the iron oxide buildup, is designed to evaporate completely. Be sure to let the fluid evaporate before you insert your own disks so that it doesn’t dissolve your diskette surface!
Don’t use rubbing alcohol (isopropyl) with a cotton swab. This is widely recommended but you shouldn’t do this. The alcohol may not evaporate completely. It may leave a residue on the drive head.
If you use your drive very much, then you can clean it once a week or so.
If you’re having problems with the drive, try cleaning the head. If you still have problems, then it’s most likely a head alignment problem. (See the previous section.)
The Hard Disk
A hard disk is like the disk drive. There is a disk and an arm with a magnetic head. Actually, there may be several heads on both sides of the disk. The difference is the very high speed of the hard disk.
The disc is metal and is sealed within the drive case. It’s usually inside your computer cabinet. It spins between 3,000 to 6,000 revolutions per minute, or about ten to twenty times as fast as a diskette. Because it spins much faster than a diskette drive, it reads and writes files quicker.
The disc is not much bigger than a diskette but it can hold much more. If a diskette holds 1.44 MB (or about 700 pages), then a hard disk can hold 500 MB (or about 250,000 pages of text) or more.
Since the metal disc is sealed within a drive, your files are also much safer. It’s rare that a hard disk loses files.
The hard disk also has a drive head, just like a disk drive. On the hard disk, however, the disk spins at about 60 miles per hour, which creates a cushion of air. The head floats above the disk. There is no wear of the disk.
Don’t Format Your Hard Disk
Don’t ever reformat your hard disk. You’ll lose every thing on it, including Windows. It will take a long time to reinstall everything.
Normally, you shouldn’t ever have to reformat your hard disk. In fact, you’ll probably never format a hard disk. If you think you need to reformat it, talk to your hard disk’s technical support people first.
Don’t worry too much about remembering this: your computer will ask you if you’re sure, if you’re really sure, and whether you have really understood this.
How to Make Backups: Disks and Tapes
Computers are not reliable. You can lose your files in many ways: electric shock, a virus, burglary, fire, etc. Of course, you can also make a mistake as well. If you use your computer for work, your hundreds of files, documents, style sheets, templates, etc. will take weeks or months to recreate. It will be very expensive to spend several weeks recreating your files.
You have to back up your files onto another media (diskettes or tape) regularly and systematically. It’s a sad fact that people only begin to make backups after they’ve lost everything.
You don’t need to back up everything. Your programs, for example, are on your original disks, so you only have to reinstall those. You only need to back up your work files, such as documents, spreadsheets, drawings, etc.
You can also back up your settings for Windows, etc. These tell Windows, Microsoft Word, DOS, etc. how you’ve configured your programs. Just copy all files with the extension GRP (for Windows groups), INI, SET, BAT, and SYS. You should also back up your WORD template files, which use the extension DOT, and your custom dictionaries, which use the extension DIC.
If you have DOS 6.0 or higher, it’s easy to back up your disks. Just type MSBACKUP. A menu appears which lets you select the type of files which you want to back up. For more information, at the DOS prompt type the command HELP MSBACKUP (and press Enter).
Does Windows take two minutes to start on your computer and nine seconds on another computer? There’s a reason for this.
As you work on your computer over several months, writing and deleting files, the hard disk turns into a jumble of files. This is called fragmentation.
Let’s say that you write two short letters. You delete the first one. On the hard disk, this leaves an empty space, followed by the second letter. Now you write a long report. The hard disk will store the first part of the report in the small space where the first short letter was, and the rest of the report will be placed after the second letter. This breaks up, or fragments, the long report.
Over the months, as you save and delete files, there will be many open spaces on your hard disk. The parts of a ten-page letter may be stored in a dozen places. When you open a letter, the hard disk’s head has to skip around the surface of the hard disk, looking for the different parts. There are a lot of spaces because a hard disk usually has some 300,000 storage slots or more.
Defragmentation is the process of reorganizing the files so that they are all in continuous threads. The hard disk reads your file quicker because it reads it in one long strip instead of having to hunt for each piece. The computer will start faster and your programs will work quicker. If you’re using diskettes, defragmentation will improve things remarkably. This is done by using the DEFRAG command in DOS.
Defragmentation Doesn’t Always Help
When you start DEFRAG, it may tell you that the disk is 67% fragmented. Is this good or bad? Should you defragment? Will it help?
There’s no clear answer. It depends on the program that you’re using. If you’re using a graphics program and color photographs, then the files are very large and the hard disk will take a long time to read or write the file. In this case, fragmentation can be a problem. If you defragment, it’ll help.
If you’re using a program which reads and writes very short files, such as a database, then fragmentation won’t be much of a problem. The head is skipping around anyway to find the small files.
The size of your cache will also make a difference. If you have a large cache, then you can continue working while the hard disk will take its merry time to write files.
You can always try DEFRAG and see if it helps.
How to Use DEFRAG
- First use Diskscan. Select Start | Programs | Accessories | System Tools | Diskscan. Set to Thorough and let it clean up your harddisk.
- Now select Start | Programs | Accessories | System Tools | Defrag. Start it and walk away from your computer.
- The first time you run Defrag, it can take all night. Afterwards, it takes only a few minutes.
How Much Room Is Left on My Disk or Hard Disk ?
If you’re using DOS, then switch to the disk drive (type A: or B: or C: and press Enter) and then type CHKDSK (and press Enter). A message will appear. Somewhere it’ll read … bytes available on disk. If it reads 17675400, then that’s 17,676,400 bytes, or about the same as 18 MB.
If you’re using Windows, then open the File Manager. Select Window | New Window so that at least one window of files appears. Select Options | Status Bar so that the status bar appears at the bottom of the screen. Click once on the drive icon at the top of the window. At the bottom, the status bar will report on the total size of the hard disk and the free space.
Getting Rid of Files You Don’t Need
If you’re running out of space on the hard disk, you can delete unused files. There are several things to do to make more space:
- Use File Manager | Files | Search to search for WBK and BAK files. These are backup files. After you’ve finished working with a project, you can delete these.
- You can delete the help files (HLP) after you’ve gotten to know Windows, Microsoft Word, etc.
- Delete unused fonts (TTF and FOT files), wallpaper (the BMP files in Windows directory), and unused accessories.
- If you’ve checked your hard disk for viruses with Windows Anti-Virus, you can delete the hundreds of small virus test files. These are called CHKLIST.MS and are found in every directory where there are programs. It’s not uncommon to have a couple of hundred of these. Use File Manager | Search to search for these and delete them all. You can always test new programs one at a time.
- Take a look at your programs. Are you using all of them? Delete the ones you don’t use.
- Another minor problem is the lost allocation files. Your hard disk leaves bits and pieces of files laying around. From the DOS prompt, type the command CHKDSK /f (and press Enter). When it finishes, type DIR (and press Enter). A number of files appear on your root directory named FILE0000.CHK, FILE0001.CHK, FILE0002.CHK, and so on. Delete all of these by typing DEL *.CHK (and press Enter).
- Windows creates many TMP (temporary) files, which it usually deletes when it closes. Sometimes, however, these aren’t deleted. These can only be deleted when you are not using Windows. From the DOS prompt, go to the TEMP directory (it’s usually in the root directory or in the DOS directory) and delete all of the TMP files.
- Print the SYSINI.WRI, WININI.WRI, README.WRI, and any other WRI (Microsoft Write ) file and then delete them from the hard disk.
- Use File Manager | View | Sort by date to sort the document files. You can see which files you haven’t used lately. Either copy those onto diskettes or delete them.
Add Another Hard Disk
If your hard disk has run out of space and you can’t delete any more files, then you can add a second hard disk. Any computer store or consultant can help you with this.
Compressing Your Hard Disk
If your hard disk is full, you can either add another hard disk or you can compress your hard disk files so that they fill less space.
Parking the Head
On older computers, you had to park the hard disk’s drive head. This meant that you couldn’t just turn off the computer. The hard disk’s drive head had to be placed in a parking track, where it could rest against the disk, without damaging any files.
On modern hard disks, namely those built after 1990, the hard disk’s head is automatically moved over to the parking sector when you turn off the computer. Don’t worry about parking the head.
Head Crashes and UFOs
There are lots of stories about hard disk crashes. One story tells how a six-micron smoke particle moving at over 90 mph slams into a hard disk head which is floating some 2 microns above the disk surface. There are cartoon illustrations which show that nasty ol’ smoke particle about to hit your nice expensive drive head.
All of this is hype. First of all, smoke won’t get into the hard disk. Most people believe that the hard disk is sealed or enclosed in a vacuum. Not true. The hard disk breathes. It also has air filters which keep out the dust particles. You can blow cigarette smoke at it. Your lungs may suffer, but not the hard disk.
Another very popular story warns that if you kick your desk, it will cause a head crash. The head digs into the surface like scooping out ice cream. Sounds great! Sorry, boys, it jes’ ain’t so. Kicking the desk won’t cause a head crash. Seagate hard disks are used in army tanks. Those 70-ton puppies go about 60 miles an hour over hill and rubble. The tank’s on-board computers and hard disks just keep humming away.
Why won’t this hurt the hard disk? The hard disk platter is spinning at about 60 miles an hour with the head about 2 microns above the disk (a micron is ten-thousandth of a centimeter). This creates a cushion of air. The closer the head gets to the disk, the stronger the air cushion. The head just can’t touch the disk surface. (This also proves that a hard disk is not sealed in a vacuum. If there were a vacuum, there wouldn’t be an air cushion.) Hard disks keep working up to 10 g’s of force. An astronaut or roller coaster passenger experiences only about 3 g’s.
If you tip over your cabinet and let it fall flat on the desk, this is about 2g’s of force. It may hurt other things, but it won’t cause a hard disk head crash.
If you want to make a head crash, you’ll need to throw the computer off a building or tease an mule. Anything less won’t hurt the hard disk.
CD-ROM Drives and CD Discs
A CD-ROM (Compact Disc Read Only Memory) drive is nearly the same as your CD music player. CD discs use a series of small pits to note the information. A laser in the drive reads those pits. There is no contact, so the disc and drive last a very long time. There is no need to clean anything.
The disc holds about 640 MB of data, which is about 250,000 pages of text or some ten thousand images.