Re: Platinum 5.25" drive problem

[david@uow.edu.au (David Wilson)]

mjmahon@aol.com (Michael J. Mahon) writes:
>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.

This is incorrect. All Mac and PC 3.5" drives are 80 track. All that
changes is the bit density (and number of sides in the case of old 400KB
MFS disks). The 1.2MB HD 5.25" drive did have this problem when writing
to 360KB DD 5.25" disks. 1.2MB drives are 80 track, 360KB/180KB drives
are 40 track.

>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

> Email:  mjmahon@aol.com
> Home page:  http://members.aol.com/MJMahon/
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia