Re: Formatting HD disks as 800K volumes

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

schmidtd <schmidtd@my-deja.com> wrote:

> On Aug 27, 12:56 am, A2Pro <a...@wilserv.com> wrote:
> >     I took the HD disk I formatted as an 800K volume using ADTPro
> > 1.0.2 to a Mac to read it using the "IIe" emulator.  But when I
> > inserted it into a 520c Powerbook, it was NOT recognized and I was
> > given the option of either formatting it or ejecting it.  I ejected it
> > and tried again with the same result. So I thought that you really
> > can't use HD disks as 800K volumes.
> >
> >     But then I tried covering up the hole on the left side of the disk
> > and reinserted it into the 520c.  Viola, it was recognized as a ProDOS
> > volume.  A bit of a surprise...
> Some drive mechanisms used the HD hole to sense (and to expect) HD
> media and HD formatting.  A ProDOS filesystem wouldn't be expected on
> HD media

Why not? I have a SuperDrive on my Apple IIgs, and some high density
ProDOS formatted 3.5" disks.

> so it decided it was unreadable... until you covered the
> hole.  So it's a combination of drive mechanism and Mac OS that gave
> you the result you saw.

A double density 3.5" drive doesn't know about the extra hole so it
assumes all disks are 800K (or 720K for PC drives).

A high density drive uses the hole to determine the expected format, so
if you cover it you can trick the drive into formatting a high density
disk as 800K (or 720K).

This is risky, because the magnetic properties are different for double
density and high density, and a HD disk written as if it was DD won't be
very reliable.

-- 
David Empson
dempson@actrix.gen.nz