#: 12189 S11/OS9/6809 (Non-CoCo)
    11-Sep-91  20:54:42
Sb: #12187-#80/40 tracks
Fm: James Jones 76257,562
To: Steve Wegert 76703,4255 (X)

You can read and write 40 track disks on an 80 track drive...MOST of the time.

What you DO NOT want to do is write on a 40 track disk that's been written on
with a 40 track drive with an 80 track disk.  Say what?!

I'll be clear this time. :-)  Consider this (as REM would say): you format a
40-track disk on a 40-track drive.  Formatting it requires writing on it... at
least writing E5E5E5E5...all over it, and a allocation bitmap, and LSN 0, and a
mostly empty root directory.

OK, you take this disk over to an 80-track drive.  It reads it just fine... and
then you write on it.  The 80-track drive has a read/write head half as wide as
a 40-track drive--makes sense, right?  So, the 80-track drive is really only
writing, where it writes, on half the space the 40-track drive wrote on. 
Everything is hunky-dory...

..until you bring the disk back to a 40-track drive and try to read it.  In the
spots the 80-track drive wrote on, it's half the old stuff that the 40track
drive wrote and half the new stuff that the 80-track drive wrote.  The 40-track
drive gets heavily confused, and it's read error time.

So, the rule is this: if an 80-track drive EVER writes on a 40-track disk, the
ONLY drives that can write on it are 80-track drives.  If that's inconvenient
or won't work for what you want to do, you'll need both kinds of disk for
writing purposes.

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