Re: Help with Apple II+ and Disk ][

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

Chad Choi Lin <chadlin@nyetcom.com> wrote:

> Romeo wrote:
> 
> > Hi. I didn't realize you couldn't use 5.25" HD disks in the Apple 5.25"
> > drive(even if they're not formatted yet?). I figured you could just pop in
> > a new HD disk and format it as a DD?
> 
> I actually tried that once -- inserted a blank HD 5.25" disk and
> attempted to format it as a DD disk -- and, believe me, it doesn't work!
> However, I am at a loss to explain *why* that should be true. For the
> edification of the newsgroup at large, could someone provide us with a
> answer to this puzzle?

This has been discussed quite recently.

I don't know the technical descriptions of what is involved, but the
basic principle is that high and double density 5.25" disks have quite
different magnetic properties: the high density disk holds a much weaker
signal than a double density one, and the drive behaves accordingly.

If you use a double density 5.25" disk in a high density drive, and the
drive tries to write to it as if it was a high density disk, the field
generated by the write head is too weak to affect the signal already
present on the disk.  (The software will usually detect this and tell
the drive to try double density instead.)

If you use a high density 5.25" disk in a double density drive, the
field generated by the head is too strong, and the drive media only
records a weakened version.  When this is read back again, it is too
weak for the double density drive's head to read.

> > Does this also apply to 3.5" HD's in the Apple 800k drive? If so, I
> > guess I'd better stock up on DD's while there's still some around!

It is true to a lesser extent for 3.5" disks, because there isn't such a
large margin between DD and HD 3.5" disks.

In practise, it seems that a double density 3.5" drive can successfully
read and write high density 3.5" disks, BUT the end result is likely to
be much less reliable than a genuine double density disk.  The signal is
likely to degrade, and the disk will lose its contents faster.

I wouldn't recommend doing this for anything more than a quick transfer
operation.  Use real double density disks for anything you want to keep.

Another issue is the extra hole in the high density disk.  If you have a
high density disk formatted by a double density drive, then try to use
it in a high density drive, the drive and/or computer will try to read
it as if it was formatted in high density, and this will fail.  The
solution to this problem is to cover up the high density detection hole,
and an opaque label is needed with some drives (possibly even opaque to
infra-red).

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand