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From: hans@umd5.umd.edu (Hans Breitenlohner)
Newsgroups: comp.sys.atari.8bit
Subject: Re: Disk R/W problem - causes?
Message-ID: <2030@umd5.umd.edu>
Date: Wed, 11-Nov-87 14:07:15 EST
Article-I.D.: umd5.2030
Posted: Wed Nov 11 14:07:15 1987
Date-Received: Sat, 14-Nov-87 02:49:15 EST
References: <1448@bsu-cs.UUCP>
Reply-To: hans@umd5 (Hans Breitenlohner)
Organization: University of Maryland, College Park
Lines: 25
Summary: Sounds like drive speed problem

Unfortunately your problem does not sound like one you can fix by cleaning
the head.  If you can read old disks, but newly written ones become
unreadable and unwritable, I would strongly suspect that your drive speed
has drifted out of tolerance.  Atari packs things slightly more densely
than others, and as a result drive speed is more critical.
If your drive has speeded up, writing a sector may cause the sector id field
of the next physical sector to be trashed.  The next physical sector usually
is 2 logical sectors beyond the current one.  (Sectors are arranged in the
sequence 1,3,5,...,15,17,2,4,6,...,14,16,18 -- at least on standard Atari
drives).
This may also explain why lower sectors work better:  The disk may be
slowed down more when the head is near the outside of the disk.

Another possibility is that the part of the drive electronics which turns
write current on and off is not functioning properly.  
This is what I would suspect if writing one sector makes more than it or the
next sector on the track unreadable.

There are a few experiments you could do.  Get a program which checks your
drive speed.   If you can find an old disk that
you don't care about, write single sectors and see how much of the track
gets trashed.  Also try formatting a new disk, and run read-write tests on
it.

Good luck, you might need it on this one!