Re: Disk transfers help please

[Michael Deutschmann <michael@talamasca.ocis.net>]

On Sat, 9 Jul 2005, Simon wrote:
> 2/ Most of the B-Sides seem to be unrecognized on the 5.25 PC drive -
> once I have inserted the disk, the drive never stops, and the transfer
> tells me all the sectors are bad. Am I doing something wrong ?

This probably has nothing to do with the fact that you are dealing with
Apple disks -- rather it's that PC hardware doesn't like flipped disks
period.

Years ago, when my family was transitioning to PC from Apple, I was
frustrated that I could not do a single-side PC format of the unused back
side of a disk (the front side being in use by the Apple).  I strain to
recall, but I think the symptoms were similar to yours -- a long spin, and
eventual errors.

I think the problem has to do with the index hole -- the little hole in
the envelope which is next to the large round hole where the motor grips
the media inside.  This hole is aligned with a second hole in the media
itself -- the two holes match up once per revolution, and some hardware
can detect this.  Some computer systems used the index hole to find
sectors based on the specific position on the disk surface.

The index hole is through both sides of the disk envelope (so it can be
read by a light beam), but it is asymmetrically positioned, and most
floppies only included a single hole.  So if you insert the disk upside
down, the index hole will be on the left side instead of the right, and
the drive will be unable to see it.

The Disk ][ doesn't use this feature, so Apples work fine with flipped
disks (assuming the media itself is OK on the flipside).  PC's apparently
freak out, because the drive mechanism doesn't see any pulses from the
index sensor when the drive spins.  It looks to the hardware as if the
motor is broken.

---- Michael Deutschmann <michael@talamasca.ocis.net>