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From: floyd@ims.alaska.edu (Floyd Davidson)
Newsgroups: comp.sys.3b1
Subject: Re: Do you use MS-DOS format floppies?
Message-ID: <1991Mar16.052401.20267@ims.alaska.edu>
Date: 16 Mar 91 05:24:01 GMT
References: <1991Mar14.212645.10354@ssd.kodak.com> <6222@amc-gw.amc.com>
Organization: University of Alaska, Institute of Marine Science
Lines: 87

In article <6222@amc-gw.amc.com> jwbirdsa@europa.amc.com (James Birdsall) writes:
>In article <1991Mar14.212645.10354@ssd.kodak.com> staffan@phos.serum.kodak.com (Kenneth Staffan (x37507)) writes:
[...]
>>9 out of 10 times (or more, it's very frustrating) when I write the disk on
>>the PC and try to read it on the 3b1, I get a "can't read file allocation
>>table" error.  If I take the floppy back to the PC, it reads it fine.  This
>>happens whether I use a freshly formatted floppy, or one which I just used
>>for a 3b1->PC transfer.  Anybody have any suggestions?
>
>   Since you have a PC-AT, I suspect that your floppy drive is high density
>(1.2M). These drives can write low-density (360K) floppies, yes, but there
>is a catch: the head is only half as wide, and the write current (and hence
>field strength on the disk) is much lower.
>
>   The head width causes problems like so: if you have written to the disk
>with a 360K drive, there is data across the whole track. When the high
>density drive writes, it writes a strip down the center of the track,
>leaving the old data around the edges. This old data can potentially
>overwhelm the desired signal and you get read errors.
>
[...]
>Even with a freshly formatted floppy, the head width can cause problems
>by corrupting the file allocation table.
>
>   HOW TO GET AROUND THIS: for transfers from the PC/AT to the 3B1, use a
>virgin disk (or one that you have bulk erased) that was formatted to 360K
>on the PC/AT. Write to this disk ONLY with the high-density drive, and there

To sum this up, if you even one time write the disk with the wide track
head of a low density drive, you can never expect to successfully
write it with a high density drive.  (The high density drive will be
able to read what it wrote, but the low density drive will get errors.)

What I wanted to add to this has to do with bulk erasing.  If the disk
has ever been written to with a low density drive it must be bulk
erased to be used by a high density drive reliably.

Even though the high density drive will probably read what it wrote,
the head alignment becomes very critical, and with only a slight
mis-alignment even the hd drive will be reading errors from the
wide track written by the ld drive.

Commercial bulk erasers probably do a good job.  I don't know because
I haven't used one since zapping 2" video tape with one in '66...
What I do use is two LARGE speaker magnets.  I put one on each side
of the disk and move them around the entire disk.  I do one at a time.
These magnets are about 4" in diameter and almost 1" thick.  Anything
less, even only one magnet, won't do it well enough!

The fact that it takes a huge magnet to really zap the disk is a
bit surprising.  Even a small magnet will destroy enough data
that the disk is useless, but for this it needs to be *totally*
zapped.

  The way I discover how much it takes is interesting.  I had a
Kaypro-4 in 1983, which would write single or double sided disks, and
automatically detected which it was when reading them.  The difference
was detected by trying to read the second side.  If it didn't get an
error it was a double sided disk, if it got the error then it was
single sided.  Now take a previously double sided disk and try to erase
it well enough that it could be formatted again as single sided...  It
has to be well enough zapped that absolutely nothing shows up on the
second side, or else the formatting goes just fine and you end up with
a double sided disk formatted for one side only.  A genuine single
sided drive can use it, but a double sided drive can't.  So you write
to it with a Kaypro-2 (SSDD) and it can read it fine.  But the Kaypro-4
couldn't use it at all (DSDD drives).

One large magnet would almost always erase the disk well enough,
but one out of ten or so wouldn't be useable.  With two magnets
it never failed.

If you want to mix and match high density and low density disks,
or 48 tpi and 96 tpi disks, get a couple of the biggest magnets
you can find.  Give every disk a good once over before re-formatting,
and plainly mark which kind of drive it was formatted on.  Then
do not ever write to it with a different type of drive.  Making
sure that no part of the disk is written to just about requires
that it be write protected whenever it is inserted into a drive
of a different type than the formatting.

Floyd


-- 
Floyd L. Davidson  |  floyd@ims.alaska.edu   |  Alascom, Inc. pays me
Salcha, AK 99714   |    Univ. of Alaska      |  but not for opinions.