Re: Shocked the Shit Outta Me!

[dempson@actrix.gen.nz (David Empson)]

Michael <michael@apple2world.com> wrote:

>  I made a pleasant discovery last night while playing with my IIgs....
> I was told by many that you CANNOT use regular 3.5' disks on the IIgs
> (the kind they sell today, high Density, IBM formatted)...

This isn't strictly true.  You can do this, but only if you like losing
your data.


With 5.25" disks, the high density and double density media are
significantly different.  A high density disk will rarely (if ever) work
if you attempt to format it in a double density drive.

With 3.5" disks, the high and double density media are not as far
removed from each other.  It is possible to use a high density disk in a
double density drive, but the resulting disk will be much less reliable
than a genuine double density one.

Under the hood, what happens is that a high density drive uses a weaker
magnetic field to write to a high density disk.  (When writing to a
double density disk, the same drive would use a stronger field, as would
a double density drive).

The disk media is constructed to retain the magnetic polarisation of the
corresponding field, i.e. a high density disk is high density disk will
retain a weaker "signal" than a double density disk.

If you write to a high density disk in a double density drive, then try
to read the disk back again, it might not be able to read the
unexpectedly weak signal.  In addition, as the signal gradually fades
over time, a high density disk written in this manner will become
unreadable (develop errors) sooner than a double density disk would.


Another issue with 5.25" disks is that high density disks have twice as
many tracks (80 instead of 40), which means that high density drives use
a narrower head.  This means that double density disks written in a high
density drive may not read as reliably on a double density drive.  (I
have observed this behaviour with PCs.)

This doesn't apply to 3.5" disks, because both densities have 80 tracks.
High density doubles the number of sectors per track.