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Hard Disks - The Essential Accessory

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A simple observation: the first accessory any computer user should buy is hard drive. On a dollar for dollar basis nothing speeds up processing and expands convenience like a hard drive. The bad news? The substantial storage capacity of a hard drive contains the seeds of data catastrophe if you don't understand how to CAREFULLY maintain a hard drive. Some reference information pertaining to larger desktop hard drives as well as smaller laptop drives has been retained since drives in both computers are similar in function although different in form and size.

Many computer operations tend to slow down at the critical bottleneck of information transfer from computer memory (RAM) to disk. The faster the transfer, the faster the program operates. Nine times out of ten it is the bottleneck formed when information flows to or from a disk that you and your program must wait. This is where a hard drive really shines - speed.

Given the best possible treatment, a hard drive should last from eight to fifteen years. Drive manufacturers typically suggest 30,000 to 70,000 hours of routine life for a hard drive before failure. If you kept your PC on for a 40 hour work week for 50 weeks - you could expect about 15 years of service for a drive rated at 30,000 hours. Some hard drive users even suggest leaving the drive on continuously or alternatively turning it on in the morning and off at night to minimize motor and bearing wear since it is the starting shock which wears most heavily on a drive. However, given marginal treatment or abuse, you can expect about fifteen minutes of service followed by a $250 repair bill. Obviously a little information about hard drives and their care can't hurt.


What is a hard drive? If you have worked with a floppy disk you already understand something about hard drives. Basically the hard drive unit is a sealed chamber (sealed against dust and dirt) which contains rapidly spinning single or multiple stacked platters. The platter(s) are similar to a floppy disk in that they store information magnetically - data can be erased and rewritten as needed. The trick is, however, that the storage capability is immense on a hard drive.

A floppy typically holds about one third of a million computer characters (360,000 or 360K bytes). The hard drive can commonly hold 20 to 40 million (or more!) bytes or computer words. In addition, the hard drive motor spins the magnetic platter quickly so that information is transferred rapidly rather than the tedious rate of the leisurely spinning floppy. A small read/write head hovers and moves above the hard drive magnetic platter much like a phonograph needle above a record. The difference is that the read/write head of the hard drive rides slightly above the platter on a thin cushion of air. In the floppy drive mechanism, the read/write head is in direct contact with the floppy. All hard drives are installed in two parts: the drive (a box containing the disk and read/write head) and the controller (a circuit board) which may be integrated into the drive or a separate circuit board. The hard drive stores the information. The controller assumes the role of a high speed "translator/traffic cop" to help the hard drive move its massive amount of information smoothly.

Back to the magnetic platter for a moment. The read write heads are mounted on a moveable arm and each position of the head above the platter defines a circular TRACK just like the track of a phonograph record. As the arm changes positions, different circular tracks are traced magnetically upon the surface of the platter. Most hard drives have several read/write heads which service both the top and bottom of each platter. A set of tracks on different platters define a vertical CYLINDER somewhat like the surface of a tin can whose top and bottom are missing. Large hard drives can have six or more platters and therefore 12 or more sides for information storage. The tracks can also be defined as divisions of equally divided data called SECTORS which are something like portions of the outer edge of a circle. Finally, the sum collection of tracks, sectors and cylinders define the entire VOLUME of the hard disk.

Each piece of data has an address which tells the read/write heads where to move to locate that specific piece of information. If you tell the read/write heads to move to and hover over a specific track, sooner or later your data will pass beneath it. Since you can move the heads directly to a given track quickly, the early nomenclature for a hard drive was the DASD or DIRECT ACCESS STORAGE DEVICE.

Movement of the read/write head arm takes a little time. For this reason an ACCESS TIME is associated with hard drives and stated in advertising and specification sheets. Generally this time is stated as the AVERAGE ACCESS TIME and is frequently in the thousandths of seconds or millisecond range which is fast indeed. The old IBM XT class machines featured access times around 85 milliseconds with the AT class machines featuring access times around 40 seconds. Newer hard drives post times in the 28 to 15 millisecond access range. Remember, the faster you can move the read/write heads, the faster you can get to your data.

The AVERAGE WAIT TIME is a less frequently discussed number but equally interesting. Once the read/write head is positioned over the track holding your data, the system must wait for the correct sector to pass beneath. Obviously, the average wait time is one half the time it takes for a full rotation of the platter. This figure is rarely given in advertisements and is usually comparable for most drives of the same type and is generally much shorter than the access time. Speed matters to a hard drive! Average wait time is published if you dig it out of the specification sheet or write to the manufacturer.

An extension of this logic brings us to consider the INTERLEAVE FACTOR for a disk. Generally a hard drive reads and writes information in sectors of the same, repeatable size such as 512 bytes. However programs and data files are usually much bigger than this and obviously must be scattered onto many sectors. The problem is that the disk rotation is much too fast for a large file to be written in perfectly contiguous sectors on the same track. If you tried to write the data onto a track, one byte after the next, the central processing unit chip or CPU could not absorb the data fast enough.

The solution is to place sectors to be read in ALTERNATING fashion which gives the CPU time to digest the data. Thus if a circular track on the platter had 8 sectors you might number and read them in this order: 1,5,2,6,3,7,4,8. This way the CPU has a "breather" in between each sector read. The number of rotations it takes the heads to read ALL tracks in succession is the INTERLEAVE FACTOR. Slow CPU chips can force a disk to use an interleave factor of 3 or even 4. A faster processor might be able to handle a disk interleave of 1:2 (such as 80286 processor chips) or even 1:1 (such as 80386 processor chips.) It is possible to low level format a disk and change its interleave factor; but if the CPU cannot keep up, the adjustment is worthless. To the processor operating in millionths of a second, the time drain of waiting for a hard drive which operates in thousandths of a second or floppy drive which operates in tenths and full seconds is wasted time. The obvious point of logic is that when using a hard drive you need to organize files for minimum time delays for the processor.

The first outer track on a disk is always the boot record which loads the main portions of DOS into the machine. Following this is the file allocation table or FAT which we discussed in earlier tutorials. The FAT maintains data in CLUSTERS which, for an XT class machine are 4096 bytes. On the AT class machine the cluster size is 2048 bytes which is much more efficient and less wasteful of disk space. Following the FAT are the sectors for the root directory of the hard drive. Each directory entry is 32 bytes in length. Curiously, and to our good advantage, unused entries in the directory have a unique first character byte. When a file is deleted though DOS, ONLY the first character is reset.

Fortunately this allows various utility programs to attempt to recover the deleted file since ONLY the directory data is altered but NOT the file itself. However, as time goes on and additional files are added to the disk, the original file is overwritten by new information. This is why you need to act immediately if you discover you have accidentally deleted a file. An advantage to the use of the FAT is that files do not have to be given a fixed amount of space on a disk - they can use as many or few clusters as needed. The downside is that the file pieces can be scattered wildly over the surface of the disk in a non contiguous fashion which only the FAT can track. This means more read/write head motion and more wasted time as far as the CPU and the performance of your program is concerned.

Additionally, if you have many deleted files within the directory, DOS must search tediously through each one from top to bottom of the directory to find a match for the file you are trying to locate. Obviously, then, programs and data of high use should have their directory entries located near the top of the directory to speed the search. Each time the read/write head moves takes time: searching the directory and finding the pieces of the scattered file all take movement of the read/write arm. There are several ways to unfragment files which boost disk performance, and we'll talk about those techniques it a bit.


Before we examine methods for improving hard drive performance, several simple "care and feeding" precautions should be mentioned.

Hard drives are touchy if mistreated! Once brought up to speed, a hard drive should never be bumped or moved. The read/write head (similar to the phonograph needle resting on a record) will smash or chip into the surface of the spinning hard drive platter and take your data with it. Either the head or the magnetically coated platter can be permanently damaged. Allow the hard drive to some to a complete stop before moving the computer.

In addition always use a "parking" software package to move the read/write head to the safety zone before turning off the computer. A parking program usually accompanies most computers which have hard drives installed or can be obtained from commercial or shareware sources. A few drives automatically park the heads when turned off but this tends to be a rare feature seen mostly on high priced hard drives.

Always maintain copies of data and programs outside the hard drive by "backing up" onto a floppy or tape. How often should you back up your files? Daily if you use the computer to produce many changes to important documents. Weekly backup is probably a bare minimum considered reasonable for occasional computer users. Other computer users maintain vital data on floppies or other backup systems and use the hard drive to store programs or applications only such as a spreadsheet or database. Backups are a good idea even for floppy disk systems which have no hard drive.

Make two copies of every file regardless of whether you have a hard drive or not. Some shareware and commercial utilities ease the backup chore by only copying those files to a floppy which have been changed or updated since the last backup has been performed. They ignore files which have not changed and thus do not require copying again. This can save a lot of time when backing up valuable files from your hard drive to a floppy for safekeeping.

Hard drives should periodically be reorganized (files unfragmented) to ensure speedy retrieval and access to data. Inexpensive or free software programs known as "disk file unfragmenters" do this job nicely. As disk files are created and deleted, blank spaces and unused sectors begin to build up.

Gradually files are broken into pieces and scattered over the many tracks and sectors of the disk. This happens to both floppies and hard drives, but is especially annoying on hard drives because of the dramatic increase in time it takes to load a program or data file. The File allocation table is the culprit, sense all data is packed away in the first and handiest sector on the drive which the FAT can find.

The FAT allows files to be fragmented down to the cluster level. One way to unfragment a disk is to copy all of the files off to floppies and then recopy them back to the hard drive - a tedious nuisance at best. You would do this with the DOS XCOPY or COPY commands but not DISKCOPY since this would retain the tracks and their fragmentation as you first found them.

Defragmenting programs perform this task without requiring removal of the files from the hard drive. They perform their magic by moving around the clusters of a scattered file in such a way as to reassemble it into contiguous pieces again. Some customization is permitted with the more sophisticated "defragmenting" programs. For example, subdirectory files can be relocated after the root or below a different subdirectory or, in another example, high use files might be placed higher in the directory listing for faster disk access.

The first time a defragmenting program is run may require several hours if a hard drive is large and badly fractured with scattered files and clusters. It is a good idea to backup all essential files prior to "defragging" just in case there is a power failure during a long "defrag". Subsequent runs of the "defragger" produce runs of only a few minutes or so since the heavy work was done earlier. Essentially, "defragging" the hard drive should be done regularaly, perhaps weekly. Defragging is not a substitute for caching, ramdisks, or buffer - instead it is a maintenance function which should be done regularly.

Yet another possible avenue to improve disk performance is that of changing the disk interleave factor which we will discuss a bit later in this tutorial. By way of brief introduction: the disk interleave indicates how many revolutions of the magnetic platter are required to read all the sectors of data from the spinning track. A ratio of 1:1 means all data can be read sequentially. One sector of data after another.

There is some overhead time required for the read/write head to zip to the FAT area of the disk (if it is not in a cache or buffer) to determine location of the next sector along the disk track.

For example, five clusters of data on a track might require four trips back to the FAT track to find the cluster addresses even on a completely defragmented disk. We will talk more about cluster and defragmenting a bit later in this tutorial.

Nevertheless, depending on the speed of your central processor or CPU, using a program which tests and alters the interleave factor, IF THIS CAN BE DONE, may yield better performance. Most interleave adjustment software first performs a test to determine the current interleave, the possible changes and of course how much performance time might be gained. A few of these packages can alter the interleave with the files in place but you should backup truly essential files before starting the process. Interleave factor adjustment are mainly derived from the CPU speed NOT the disk speed. Thus a fast AT or 80386 equipped machine will more likely be able to take advantage of an interleave adjustment.

Tinkering with a hard drive for optimum results might best be divided into two categories: DISK SUBSTITUTION and DISK ALTERATION. DOS allows two clever ways substituting RAM memory for disk memory.

In the first, using BUFFERS, the small CONFIG.SYS file on your hard drive is modified to contain a buffers statement. A sample might be: BUFFERS=20. A DOS buffer is an area of RAM memory capable of holding a 512 byte mirror image of a disk sector. This allows DOS to quickly search the buffer area for frequently used data instead of the slower disk. In the older XT class machine, if you did not specify a buffer size, DOS defaulted to 2 buffers while later versions of DOS default to about 10 buffers. Most users settle on about 20 buffers but you can specify up to 99 with current releases of DOS. But you don't get something for nothing. If you used the full 99 buffers available, you would soak up 45K of your main RAM memory! The downside of using buffers is that more is not necessarily better.

Unfortunately, DOS searches the buffer area of RAM sequentially rather than logically so if DOS requires data which is in the buffer area, it will search each 512 byte area in sequence from top to bottom even though the data it needs may be at the end of the buffer. Logically, then, there is an optimum number of buffers - too many used with a small program and you can slow things down, not enough and DOS will be forced to go out to the disk to retrieve what it needs. If you rarely use the same data within a program twice but load lots of different programs and data, a large number of buffers won't help. However if you need frequent access to a certain data file or portion of that file, buffers will help. Portions of the FAT are kept within the buffers area, so dropping your buffers to zero has the damaging effect that DOS must always go to the disk to read the FAT which isn't helpful either.

Another way of substituting RAM memory for disk memory involves using a RAMDISK. The idea is to create in RAM memory an entire disk or a small portion of a disk. This works like magic on many machines since the reading of tracks and sectors takes place at the high speed of RAM memory rather than the mechanically limited speed of the read/write heads on a floppy or hard drive.

But be careful. Three areas of difficulty can arise. First you must remember to take the data from a floppy or hard drive and move it into the RAMDISK. Many people do this automatically from within an AUTOEXEC.BAT file or may have several floppies, each with a different RAMDISK configuration depending on the task at hand. Copying data to the RAMDISK usually moves along briskly. Secondly you must sacrifice a large area of memory for the RAMDISK which can no longer be used by your main program. Users of computers with extended or expanded memory usually choose to put their RAMDISK in the extended or expanded memory area of RAM so that precious main memory is not lost. Still, a small RAMDISK can soak up 64K of RAM memory and one or two MEG RAMDISKS area common for many users. The third and most serious problem when using RAMDISKS is that they are volatile - switch off the machine or experience a power failure, and your data is lost forever! Rather than residing safely on a magnetic disk, the data is "floating" in RAM memory and should be - MUST BE! - written to a disk before the machine is powered down.

Many applications fly with a RAMDISK. Users of word processors find that moving the spelling checker and thesaurus to the RAMDISK speeds up things considerably since these are used heavily in a random manner. Spreadsheet users find that reading and writing short data files to RAMDISKS is a boon. Programs which use overlay files or temporary files as well as programming compilers benefit from RAMDISK use. Batch files which are disk intensive as well as small utilities really sprint when placed on a RAMDISK. Basically, any program file which is frequently used and loaded/unloaded repeatedly to a disk during normal computer operation is an excellent candidate for RAMDISK placement. DOS contains a RAMDISK which is called by using the statement DEVICE=VDISK.SYS or DEVICE=RAMDRIVE.SYS (if you are using MSDOS) which is placed in your CONFIG.SYS file. Your DOS manual details the specifics such as stating the size of RAMDISK and giving it a drive letter. You must still copy your target files into the RAMDISK and place it in the search path (with the PATH= command) as we mentioned in a previous tutorial. And the RAMDISK should always be the first drive letter mentioned in the path command so that DOS searches it first for optimum results.

Yet another area of investigation is that of CACHE software. Essentially a CACHE is an extension of the buffers idea we discussed earlier. But the twist is that the CACHE is searched intelligently by a searching algorithm within the CACHE software rather than from top to bottom as with the more typical DOS buffer search system. Disk CACHE software can be obtained as either commercial software or shareware. As with a RAMDISK, the CACHE requires a chunk of RAM memory to operate. This can be extended memory, expanded memory or main RAM memory. Some manufacturers include a CACHE program with the software package or DOS disk. A CACHE is a sophisticated type of RAMDISK, in a rough sense.

CACHE software allocates a large area of memory for storage of frequently used disk data. This data is updated by an intelligent CACHE search algorithm in an attempt to "guess" which tracks of a disk you might read or need next. The CACHE also stores the most frequently used disk data and attempts to remove less frequently used data. Whenever DOS requests disk data, the CACHE software first tries to fill the order from data currently stashed in the CACHE which prevents a slower disk search.

When data is written from the program to the CACHE, first a disk write is done to prevent data loss in case of power failure and then the data is stashed in the CACHE in case it is needed again. Usually the hard drive data is the target of the CACHE activity, but a floppy disk could also be cached. All CACHE software allows you to allocate the size of the CACHE as well as the drive or drives to be cached. And some even allow you to specify exact files or data to be cached. The key is that high use data lives in RAM memory which keeps tedious disk access times low. In general, if your computer has a megabyte or more of memory and a speedy processor such as an 80286 or 80386 either or both a CACHE or RAMDISK option does improve performance.

As we leave hard disk boot camp, let's finally look at hard drive formatting processes. Two basic formatting operations are of concern: physical formatting or low level formatting and logical or high level formatting. When you use the format program on a floppy disk both low level and high level formatting is accomplished. On a hard disk, formatting performs only logical or high level formatting. On a hard disk, low level formatting is usually done to a disk before shipment. As an aside, the FDISK command of DOS has little to do with either type of formatting, but is a method of partitioning or arranging the data onto the hard drive tracks. Each disk platter is separated into circular concentric tracks where data is stored as we saw earlier. During physical formatting the tracks are divided into further subdivisions called clusters and further yet into sectors. High level formatting involves the specific ordering of the space for the exclusive use of DOS and is a bit more analogous to the formatting of a floppy disk.

Some software programs of use by hard drive owners:

The following two programs perform low level formatting and simple diagnostic routines on a hard drive:

Disk Manager and CheckIt

Data recovery and "unerasing" programs also containing diagnostic routines are:

PC Tools Deluxe, Norton Utilities, Mace Utilities

Extensive diagnostic and maintenance/data repair functions as well as interleave alteration and head parking are offered by:

SpinRite II, Optune, Disk Technician

Shareware programs with unerase functions include:

Bakers Dozen

Shareware programs with defragmentation capabilities include:


Tutorial finished. Be sure to order your FOUR BONUS DISKS which expand this software package with vital tools, updates and additional tutorial material for laptop users!