249 lines
15 KiB
Plaintext
249 lines
15 KiB
Plaintext
EVERYTHING YOU EVER WANTED TO KNOW ABOUT FLOPPY DISKS.......
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BY Ted Jensen
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FROM: ARTICLES, "The KAY*FOG Online Magazine."
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So, you've just spent 400 bucks for that super piece of software,
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made your back-up and are working away with your working copy.
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Suddenly, while you are working with a relatively unimportant utility
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program on another disk, your disk goes bad. This is not a major
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problem. You have a back-up somewhere, but it gets you to thinking
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about your back- ups on your commercial programs. What happens if they
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go bad? Should you have made them on some type of premium diskette to
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guard against that?
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You scour through catalogs and ads in magazines. There are sources
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galore for diskettes, at all prices, and some of them even have
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specifications. You run into one spec called "Clipping Level" and the
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supplier claims that because his disks have been tested to a higher
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clipping level they are superior. Should you pay a premium for disks
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with superior specifications? What do these specs mean? Will your
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back-ups be less likely to fail if you use premium discs?
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These are difficult questions to answer. Perhaps an explanation of
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some of the tests run on disks and what can happen to your back-ups with
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time would help you make that decision. In addition, you may be
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interested in considering the cost tradeoffs of using higher priced
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disks.
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As an engineer with many years of experience in magnetic recording
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I had never heard of the term "clipping Level" until it came up in a
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discussion on KAY*FOG. In fact, I had never seen a specification sheet
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in any box or bag (I buy the cheap stuff by mail order too!) of disks I
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have purchased. However, I did spend a couple of years as a part of a
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design team on a Winchester Drive for personal computers and one of my
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tasks was the specification and testing of the disks used in those
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drives.
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CLIPPING LEVEL: Since magnetic media is pretty much the same
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whether it is tape, diskettes, or hard disks (the major difference being
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that the material to which the magnetic particles are bonded is mylar
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for tape and diskettes, and aluminum for the hard disks), it wasn't
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difficult for me to guess at what was meant by "clipping level." A
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little looking through a parts catalog and I found a specification on a
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chip designed for use in disk drives and they defined "clipping level"
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(although in rather vague terms). It is unfortunate that these words
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are used to describe a test performed on diskettes since they have a
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different and more widely understood meaning throughout the general
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electronics industry. In any case we will have to accept these words
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since they are the ones used in the advertisements.
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In simple terms, your drive uses a "head" to read the information
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on the disk. You can think of this as being like the needle and pick-up
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on your phonograph. The head reads the magnetic information previously
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written on your disk and converts it into an electrical signal. This
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signal is further processed and eventually takes on a form suitable for
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transmission to your computer as bits, or bytes, which represent the
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data.
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SIGNAL VARIATION: The size and shape of the electrical signal
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developed by the head varies for many reasons. First of all, it varies
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as a result of the information written on the disk, and this variation
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itself represents that information. However, there are other variations
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which take place due to imperfections in the head, the mechanical
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characteristics of the drive, or imperfections in the diskette. These
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variations, if large enough, will lead to the electronics in the drive
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not being able to correctly decode the information, and your computer
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will indicate by means of some error message that it cannot read the
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disk. It is therefore important to keep these variations (those not
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part of the data) at a minimum.
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COATING THICKNESS: Magnetic diskettes or tapes are manufactured by
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bonding magnetic particles to a flexible mylar backing material.
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Characteristics which affect the performance of the final product
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include, but are not limited to, the magnetic characteristics of the
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particles, the size of the particles [Note: it is modification of these
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two characteristics that make the difference between a 1.2M disk and a
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360K disk.], the thickness of the coating, and, most important to the
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subject of "clipping level," the uniformity of the coating. If a tiny
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part of the disk, the size of a pinhole, does not get coated, the signal
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level recoverable from that spot is reduced. Thus, if there are a
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number of these of sufficient size, the level of the signal will be
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fairly uniform until that "pin-hole" passes under the head, at which
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point it will drop. These are referred to as "drop-outs" in the
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industry.
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Furthermore, if the coating thickness varies over the surface of
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the disk, the amplitude of the signal can vary in a relatively smooth
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manner as the disk rotates. This is generally not a serious problem,
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however.
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Your drive can recover your data by separating these disk related
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variations from the variations in signal due to the real data, provided
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that disk related variations are not too large. Typically a drive might
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be able to successfully ignore disk related variations which did not
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reduce the amplitude of the real signal to less than 30% of the normal
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output. This number, however, also depends on a wide variety of
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factors, and varies from drive to drive, even the same model from the
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same manufacturer.
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Thus, anything one could do to assure that the level of these disk-
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related variations are held within a specified range should reduce the
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probability of errors. The key word is "probability," and more will be
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said about this later. Therefore a disk which is tested to a "clipping
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level" of 60% is tested to assure that the variations due to the disk
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are small enough that the signal level never drops below 60%. That is,
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the variations are held to a range between 60% and 100%. It follows
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that the higher the "clipping level," the less variation in signal
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output and the reduced probability of a disk error.
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Now comes the tough part. How much extra money should you pay for
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a disk tested to a 60% level as compared to one tested to a 40% level?
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Would you pay 50% more? Twice as much? Ten times as much? The way I
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look at it is this: There is a high probability that if I buy 25 or 50
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brand X disks and they all work,whatever tests were run on them were
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probably sufficient to assure me that brand X disks will always work. I
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have no way of knowing what "clipping level" disks destined for my
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drives should be tested at, nor, do I believe, do the manufacturers of
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floppy disks.
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A WORD ABOUT HARD DISKS: In the case of Winchester drives, the
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situation is a little different. The manufacturers of the disks which
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go in these drives are generally different companies than those that
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manufacturer to drives. The drive manufacturer imposes specifications
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on the disk manufacturer. Furthermore, the drive manufacturer
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continually tests disks using sophisticated equipment to be sure that
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the disk manufacturer meets these specifications. That is, people who
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manufacture disks for use in hard drives do not sell them directly to
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the end user (removable hard disks being the exception).
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BOTTOM LINE $$$: But, back to floppies. Assume I buy 100 diskettes
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from each of two sources, SuperDisk and CheapDisk. Assume I pay 40
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cents each for the CheapDisks and $2.00 each for the SuperDisks.
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Finally, out of all the disks I bought, one SuperDisk won't format and
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10 CheapDisks won't format. I have ended up paying slightly over
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$2.00/disk for the good SuperDisks and about 45 cents each for the good
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CheapDisks. I still think I got a better buy on the CheapDisks.
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MORE USE -- BETTER PERFORMANCE: Now, what about disk failures in
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the future? That is as I use these 90 CheapDisks are they more likely
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to fail in the future that the 99 SuperDisks? Well, I suppose there are
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those who would argue with me that in fact they would. But I really
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don't believe it. The reason is that the first few times I use any
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diskette its performance will improve. The surface of the disk is left
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slightly rough (not on purpose) during the manufacturing process and
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this process prevents good contact between the head and the disk. This
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poor contact degrades performance of the disk. As the disk is used and
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rotated past the head, the head knocks off some particles of the
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coating, smoothing the surface and improving the contact and the
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performance. In tape recording, in critical applications, new tape is
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never used without running it through a machine at least once and
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sometimes several times, just for this reason. Therefore, after I have
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used my CheapDisks several times I feel more comfortable with them than
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when they were brand new.
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HOW LONG WILL THEY LAST? Finally, what about the really long term?
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Will CheapDisks retain the information stored on them equally as well as
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SuperDisks, say over a period of 100 years? Well, here we are dealing
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with real unknowns. There are no disks around that are a hundred years
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old. Magnetic recording using media of the type used in disks is only
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about 40 years old. Archival data that has been around for long periods
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of time has turned out to be a problem in a number of fields. Ask a
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librarian about the problems facing the Library of Congress in
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protecting many of its books.
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There has been some experience with magnetic recording in general
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that may be of interest. In tape, such as your audio or video
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cassettes, or computer tape as used on large main frames, there is a
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problem with long term storage known as "print through." The magnetic
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pattern on the tape representing the information emanates a magnetic
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field, just as the North and South Poles do. This field is very minute,
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but still present, and any material susceptible to being magnetized will
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do so in the presence of a magnetic field. This is true even for weak
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fields if the material is held still within the field for long periods
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of time. All tape is susceptible to being magnetized, that is it's prime
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purpose in life.
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When wound on a reel, each piece of tape is tightly pressed against
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another one, and each piece emanates a field. If the tape is left
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untouched in this form for several years, a little of the information
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recorded on each piece is transferred to mix with the information on the
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adjacent piece. In audio tapes one can hear this as a low level
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background of the same music that played either a few seconds earlier or
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a few seconds later, particularly where a loud passage is immediately
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followed by a quiet one.
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Normally disks have a jacket around them that is fairly thick.
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Thus it is unlikely that print through would take place between disks.
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On double sided disks however, the magnetic information on one side is
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pretty close to that on the other side, the distance being in the same
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range as that previously discussed in the case of tape on a reel. If I
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were to make a guess at the first cause of long term failure, in the
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sense of not being able to recover 100% of the material form a floppy, I
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would guess that "print through" would be the cause.
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RE-COPY YOUR FLOPPIES: Someone once raised the question of whether
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it makes sense to re-copy masters or back-ups from time to time to make
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new backups. My initial reaction was that I didn't think it was
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worthwhile. Having given it some thought, however, it might not be a
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bad idea. If there is a degradation that takes place with time on an
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untouched back-up as it sits on the shelf, re-copying does in fact
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restore the information to a more pristine state and thus acts as added
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protection against the probability of losing your data. As to
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SuperDisks being any better than CheapDisks in an archival sense, I can
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think of no reason why there should be any difference, but perhaps we
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won't know the answer to that for another 100 years.
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HOW COME SO CHEAP? There are a lot of reasons SuperDisks sell for
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more than CheapDisks. They spend more on advertising, and packaging,
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and possibly corporate headquarters. They sell primarily to companies,
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which avoid buying anything by mail order from some post office box
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across the country. And they sell at the price they do because people
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are willing to pay for it, whatever the reason. In fact, however, if
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you look into it you will find that many of the people selling the
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cheaper diskettes are buying their raw material from the same source as
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those selling the expensive versions.
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The whole thing about mass produced products, whether it is disks,
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drives, computers, or light bulbs, is that they are produced on a
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statistical basis. That is, costs are reduced to the point where the
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probability of a bad one getting to the user is acceptably low. This is
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simply good business. No company can stay in business if it strives for
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perfection in a commercial product line. Only governments can afford
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products which have been tested to the level of a space shuttle, and as
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we found out even they are not perfect.
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Personally I have always bought the least expensive disks I could
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find. Furthermore, I buy single sided, single density diskettes and use
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them in double sided, double density drives, with no problem. On one
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occasion, I paid over $25.00 for a box of 10 diskettes. It was a
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Sunday, I needed them, and they were the only ones I could find. One of
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the disks in that box proved to be the first bad disk I ever ran into!
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One last comment on probabilities. If the probability of a given
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disk failing is one in 1,000 under whatever circumstances, the
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probability of two failing under the same circumstances is one in 1,000
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times 1,000, or one in 1,000,000. Anyone for making two 45 cent back-
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ups instead of one $2.00 back-up?
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Note: I have taken some liberties in the preceding article in the
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interests of keeping it from becoming overly technical but do not
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believe these affect the substance of the arguments for purchasing lower
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cost disks. Also, I was not able to find detailed information on the
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testing of diskettes in the literature and much of the above is based on
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extending my experience from tape and hard disks to diskettes.
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I would appreciate it if anyone having more information on the
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subject, or finding inaccuracies within the article contact me either
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through KAY*FOG PCBBS (415)285-2687 or by mail: P.O. Box 324, Redwood
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City, CA 94062. |