Re: Platinum 5.25" drive problem

[mjmahon@aol.com (Michael J. Mahon)]

Matthew Russotto replied:

>In article <20010711191747.17797.00005714@ng-cg1.aol.com>,
>Michael J. Mahon <mjmahon@aol.com> wrote:
>>Matthew Russotto wrote (in response to me):
>>
>>>In article <20010710181837.03517.00004552@ng-fi1.aol.com>,
>>>Michael J. Mahon <mjmahon@aol.com> wrote:
>>>>
>>>>In short, don't worry about using permanent magnets to disrupt the
>>>>magnetic patterns recorded on floppys (but don't magnetize the hub
>>>>of a 3.5" diskette--that's not good.  ;-).
>>>
>>>You can't damage the head with a magentized floppy.  But you can make
>>>the floppy unusable until demagnetized.  Bulk erasers use an AC field,
>>>and attenuate it gradually at the end of the cycle.
>>
>>The erase and write heads of the floppy drive are, by definition, strong
>>enough to deal with magnetic media in ANY state of magnetization.
>
>Even supposing the floppy drive fully magnetizes the part directly
>under the head, the parts just off-track will see a smaller field and
>therefore become less magnetized.  If some external source strongly
>magnetizes the off-track parts of the media, the stray fields may make
>reading and writing the floppy unreliable until it is demagnetized.
>Something like this happens when you try to re-write a low-density
>floppy on a high-density drive (even using low-density mode).

This is both true and irrelevant, unless there is out-of-tolerance
head alignment on one or both drives.  In the case of compatible
Apple drives, the head widths are all made to the same specification.
The erase head erases a little wider track than the read/write head
reads or writes, so that minor alignment tolerances are not a problem.

The only exception would be using a SuperDrive to write an 800K
floppy, in which case only about half the 800K track width would
actually be erased and written.  (This is the problem in the PC world
when an HD 5.25" drive is used to write to a diskette formatted on
a lower density drive.)  Even then, there is no problem until the disk
is taken back to a lower density drive to be read.  The lower density
drive's wider head will read both the unerased old data and the newer,
narrower data, resulting in an I/O error.

If the disk is bulk-erased (here I wouldn't recommend permanent magnet
erasure) and formatted on a HD drive, it can be read on a lower density
drive without problems, since the wider head doesn't pick up an interfering
signal away from the narrower HD track.

>>I would not depend on a monitor degausser to erase a floppy, but
>>neither would I hold a floppy I cared about near one during the
>>degauss cycle.  ;-)
>
>I've used the trick when I had floppies which would crash the machine
>(Macintosh) when inserted (due to software issues, not hardware).  A few
>zaps and they could be re-initialized.  Didn't have to erase the whole
>thing, just sufficiently corrupt the master directory block.

Yes, as you point out, the Mac comes to certain conclusions from
reading the first few blocks, and is sometimes reluctant to revise its
conclusion.  Corrupting the first few blocks keeps it ignorant and willing. ;-)

-michael

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