Re: High and Double density floppies

[nraymond@genesis.nred.ma.us]

As I understand it, high density floppy disks have a higher coercivity
than low density floppy disks.  I believe this is like resistance, and I
know that if you take a 3.5" low or double-density floppy disk and bore a
hole, melt a hole, or punch a hole where the HD hole should be and then
format it high density, it may not fail today, it may not fail tommorow,
but one day sooner than normal it will fail.  This is due to the fact that
because tracks are packed tighter on HD disks, eventually some information
"spills" into adjacent tracks on a LD or DD disk formatted HD, scrambling
the data because of the low coercivity.  And the converse is true;
formatting a higher-resistance material (needed to keep tracks from
"spilling" during writes) to a lower density won't work because the
resistance keeps the information from being recorded properly.  Apple, for
instance, prints a label on the shutters of all its HD system disks with
400k/800k circled and crossed off in red.  In any event, trying to format
disks from low or double to high or vice-versa isn't a good idea unless
you really don't like the information you're putting on them.

As for MFM and GCR encoding, true, the format types are incompatible but a
disk formatted one way can be easily formatted the other (on a Mac you can
now format a double-density disk as "HFS Interchange Format" - MFM
recording at 720k (instead of variable track length 800k).  The advantage
of Mac 720k MFM disks is that they can be read in IBM or compatible
machines with file interchange programs, just as 1.44MB MFM disks can
(Apple could have gone with variable track length HD disks but decided not
to in order to ease data interchange; they were forced to use 400k because
the original Mac 5.25" "Twiggy" drives (so bad they never were produced) were
400k and the operating system had been designed to work with 400k volumes,
the whole 3.5" Sony drive project was done in secret from the Mac project
heads and unveiled only when Twiggy proved both unreliable and nearly
impossible to produce, so Apple and Sony came up with the solution of
squeezing more tracks onto the disks so they would be the same formatted
capacity as Twiggy would have been).  An interesting note: when I take an
IBM or compatible MFM formatted DD 5.25" disk and format it with Copy II
Plus 8.4, the track seek intervals expose a residual formatting pattern,
but when the same disk is formatted with Apple's ProDOS 8 utility program,
the disk behaves normally - I know there is no ROM based formatting code
(other than a low-level command for UniDisk), so these two algorithms must
be different.  Anybody know what the difference might be?  Disks formatted
with both programs have been equally reliable for me.

nraymond@genesis.nred.ma.us