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The Logical Structure, Organization, and Management of Hard Disk Drives by Steve Gibson GIBSON RESEARCH CORPORATION 1989 When a file is deleted from a disk, its directory entry is flagged as unused and each cluster which the file occupied is flagged in the system’s FAT as being free for use. If the surrounding clusters are still in use by other files, thiscreates a “hole” of free space in the disk. Page 7

The Technology of o o Magnetic Disk Storage o oo o by o o Steve Gibson o o GIBSON RESEARCH CORPORATION 1989 Since a drive’s read/write head and disk “communicate” using magnetic fields, and since magnetic fields travel through the air readily, actual physical contact between the head and disk is not necessary. The disk drive’s head and disk only need to be close enough to magnetically “couple” and influence each other as a result.

A disk’s read/write head is a specially designed coil of wire wrapped around a metal armature. This armature has a very tiny GAP across which the magnetic field generated by the coil JUMPS. The gap serves to concentrate the jumping magnetic field into a tiny spot on the disk. As the field jumps the gap, a bit of magnetic field protrudes from the head and passes through the nearby disk or diskette. When a read/write head wears out it’s because this gap has widened, becoming too large, and thus has lowered the resolution of the head. Page 1

Since disks are composed of a metallic oxide, tiny spots of the disk become magnetized and thus “remember” the magnetic field which was imposed. Reading data is essentially the writing process in reverse. The tiny magnetic spots on the disk create their own tiny protruding magnetic fields. As the disk rotates, the disk head passes over these tiny protruding fields. When these fields fall across the gap in the read/write head a small electric current is induced in the head’s wire coil. A sensitive READ AMPLIFIER boosts this signal up to useable strength for interpretation as the data stored on the disk. Page 2

==A Hard Disk Drive o o for o o Steve’s Dream Machine o oo o by o o Steve Gibson o o GIBSON RESEARCH CORPORATION 1989 == When it wants to read or write a sector, it first determines where the sector is located on the drive then sends the heads to that location. Page 1


            FROM: ARTICLES, “The KAY*FOG Online Magazine.”

Since magnetic media is pretty much the same whether it is tape, diskettes, or hard disks (the major difference being that the material to which the magnetic particles are bonded is mylar for tape and diskettes, and aluminum for the hard disks) page 1

  In  simple terms, your drive uses a “head” to read the  information

on the disk. You can think of this as being like the needle and pick-up on your phonograph. Page 1

Finally, what about the really long term?

Will CheapDisks retain the information stored on them equally as well as SuperDisks, say over a period of 100 years? Well, here we are dealing with real unknowns. There are no disks around that are a hundred years old. Magnetic recording using media of the type used in disks is only about 40 years old. Page 3

The magnetic pattern on the tape representing the information emanates a magnetic field, just as the North and South Poles do. This field is very minute, but still present, and any material susceptible to being magnetized will do so in the presence of a magnetic field. This is true even for weak fields if the material is held still within the field for long periods of time. All tape is susceptible to being magnetized, that is it’s prime purpose in life. Page 3

normally disks have a jacket around them that is fairly thick. Thus it is unlikely that print through would take place between disks. On double sided disks however, the magnetic information on one side is pretty close to that on the other side, the distance being in the same range as that previously discussed in the case of tape on a reel. If I were to make a guess at the first cause of long term failure, in the sense of not being able to recover 100% of the material form a floppy, I would guess that “print through” would be the cause. Page 3

RE-COPY YOUR FLOPPIES: Someone once raised the question of whether it makes sense to re-copy masters or back-ups from time to time to make new backups. My initial reaction was that I didn’t think it was worthwhile. Having given it some thought, however, it might not be a bad idea. If there is a degradation that takes place with time on an untouched back-up as it sits on the shelf, re-copying does in fact restore the information to a more pristine state and thus acts as added protection against the probability of losing your data. Page 4

There are a lot of reasons SuperDisks sell for more than CheapDisks. They spend more on advertising, and packaging, and possibly corporate headquarters. Page 4

No company can stay in business if it strives for perfection in a commercial product line. Only governments can afford products which have been tested to the level of a space shuttle, and as we found out even they are not perfect. Page 4

One last comment on probabilities. If the probability of a given disk failing is one in 1,000 under whatever circumstances, the probability of two failing under the same circumstances is one in 1,000 times 1,000, or one in 1,000,000. Anyone for making two 45 cent back- ups instead of one $2.00 back-up? Page 4 –