HARD DISKS

flexible disk that bends around the head under the force of the air pressure. The advantages of this system are that the spinning disk has much lower mass and is more resistant to head crashes. The disadvan­tage is that the medium is constantly flexed and eventually wears out.

Fixed Disks

IBM, and just about no one else, calls the hard disk a fixed disk because, unlike floppy disk drives, the recording media cannot be removed from a hard disk. Strictly speaking, the disk of a hard disk is not fixed—it rotates faster than IBM can turn a buck. Moreover, a few hard disk drives use removable media.

No matter the terminology—hard disk, Winchester, or fixed disk— the technology is the same, as are your concerns in installing, using, and taking advantage of one.

Understanding Hard Disks

Not all hard disks are created equal. Different hard disk models are made from different materials using different technologies and under different standards. As a result the performance, capacities, and prices of hard disks cover a wide range from a few hundred dollars to tens of thousands. Understanding these differences will enable you to better judge the quality and value available in any disk product. You'll also better understand what you need to do to get one running and keep it that way.

The hard disk is actually a combination device, a chimera that's part electronic and part mechanical. Electrically, the hard disk performs the noble function of turning evanescent pulses of electronic digital data in­to more permanent magnetic fields. As with other magnetic recording devices, from cassette recorders to floppy disks, the hard disk ac­complishes its end using an electromagnet, called a read/write head, to align the polarities of magnetic particles on the hard disks themselves. Other electronics in the hard disk system control the mechanical half of the drive and help it properly arrange the magnetic storage and locate the information that is stored on the disk.

Platter Composition

Typically, the platters of a hard disk are made from an aluminum alloy that's precisely machined to an extremely fine tolerance measured in microinches. The aluminum serves as a substrate to which a magnetic medium is affixed either with a binder or mechanically.

Oxide Media

The first magnetic medium used in hard disks was made from the same materials used in conventional audio recording tapes, ferric or ferrous oxide compounds—essentially fine grains of rather exotic rust. As with recording tape, the oxide particles are milled in a mixture of other com­pounds, including a glue-like binder and often a lubricant. The binder also serves to isolate individual oxide particles from one another. This mud-like mixture is then coated onto the platters.

The technology of oxide coatings is old and well developed. The process has been evolving for more than 40 years, and now rates as a well-understood, familiar technology. In computer terms, most of the bugs have been worked out. In addition, many sources of supply are available. Consequently, oxide coatings are a safe bet for disk makers.

Oxide particles are not the best storers of magnetic information, however. Oxides tend to have lower coercivities and their grains tend to be large when compared to other, newer media technologies. Both of these factors tend to limit the storage density available with oxide media. The slight surface roughness of the oxide medium also requires the hard disk read/write head to fly farther away from it than other media, which also reduces maximum storage density. In addition, oxide coatings are generally soft and are more prone to damaging head crashes.

Thin-Film Media

The competing technology is thin-film magnetic media. As the name implies, a thin-film disk has a microscopically skinny layer of a pure metal, or mixture of metals, mechanically bound to its surface. These

Platter Speed Effect

The number of platters inside a hard disk also influences the speed at which data stored on the hard disk can be found. The more platters a given disk drive uses, the greater the probability that one of the heads associated with one of those platters will be above the byte that's being searched for. Of course, the more heads in an assembly, the more massive it Will be. This additional mass tends to slow things down, but it can be compensated for with a more powerful actuator.

Hard Disk Vulnerabilities

The strengths of the hard disk also have their down side. For instance, the constant spin of the hard disk platters extracts its own penalty. The spindle motor continuously consumes enough power to preclude the use of most hard disk drives in computers with modest power supplies, such as the paltry 62.5 watts of the IBM PC or the 35 watts of the PCjr.

Head Crashes

The precision mechanism of the hard disk is also vulnerable to en­vironmental damage. Shock can cause the flying hard disk head to im­pact on the media on the platters. Or contaminants such as dust or air pollution particles on the media surface can strike the head and upset its flight. The head touching and damaging the media may result in a head crash, which not only destroys the storage ability of the media in the area struck by the head but also loosens particles of media that can, in turn, cause further head crashing.

Landing Zone

Hard disks are most vulnerable to head crash damage when they are turned off. As soon as you flick the off switch on your computer, the platters of its hard disk must stop spinning, and the airflow that keeps the heads flying stops. Generally, the airflow decreases gradually, and the head slowly progresses downward, eventually landing like an air­plane on the disk media.

In truth, however, any head landing is more of a controlled crash and holds the potential for disk damage. Consequently, most hard disks

Third-Party Disk Formatting

Many of the manufacturers of aftermarket hard disk controllers include the necessary program for low-level formatting a hard disk, which is connected to their product in the controller's ROM firmware. These routines are normally executed through the Go command of the DOS DEBUG program. For instance, a number of these routines are accessed by typing the following instruction at the DEBUG hyphen prompt:

G = C800:5

If you try this with your controller and it doesn't work, you'll likely lock up your system. If it works, you'll be prompted on the screen. The built-in low-level formatting rountines of some vary a few bytes in posi­tion in their add-in BIOSs. You may want to try starting execution at C80():6 or C800:8 if the first example does not work. Other than locking up your computer, you won't do any damage to anything. In particular, you won't hurt any data on a new hard disk because there's nothing there to begin with!

Bad Sectors

In the manufacture of hard disk platters, defects occasionally occur in the magnetic medium. These defects will not properly record data. Sec­tors in which these defects occur are called bad sectors; the tracks con­taining the sectors are called bad tracks.

Your computer can deal with bad sectors by locking them out of nor­mal use. During the low-level formatting process, the sectors that do not work properly are recorded and your system is prevented from us­ing them. The only ill effect of reserving these bad sectors is that the available capacity of your hard disk may diminish by a small amount.

Some low-level formatting programs require that you enter bad sec­tor data before you begin the formatting process. Although this seems redundant (the format program will check for them anyhow) it's not. Factory checks for bad sectors are more rigorous than the format routine. This close scrutiny helps minimize future failure. Tedious as it is, you should enter the bad sector data when the low level format pro­gram calls for it. The listing of bad sectors is usually on a sheet of paper accompanying the disk drive or on a lable affixed to the drive itself.

Track 0 Bad

The only time a bad sector is detrimental is when it occurs on the first track of the disk. The first track, Track 0, is used to hold partition and booting data. This information must be located on the first track of the disk. If it cannot be written there, the disk won't work.

Should you get a hard disk with Track 0 bad, return it to the dealer from whom you bought it. If you reformat a disk after a head crash and discover Track 0 bad during the format process, you need a new disk.

Partitioning

Once the low-level format is in place on a hard disk, you must partition it. Partitioning is a function of the operating system. It sets up the logical structure of the hard disk to a form that is compatible with the operating system.

The IBM program for partitioning is called FDISK. After low-level formatting your disk, you must run FDISK before you can do anything else with the disk using the DOS (or OS/2) operating system.

DOS Formatting

The final step in preparing a disk for use is formatting it with the operating system you intend to use.

Note that IBM operating systems are backwardly compatible but not forwardly compatible. If you format a hard disk under DOS 3.3, you may not be able to use it under DOS 2.1. You will either get an error message or see strange things on your screen, such as file names con­sisting of odd combinations of numbers and smiling faces. Never write to a hard disk using a version of DOS from a previous generation to the format that's on the disk. If you do, the disk will be irreparably damaged