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From: jwbirdsa@amc-gw.amc.com (James Birdsall)
Newsgroups: comp.sys.3b1
Subject: Re: Do you use MS-DOS format floppies?
Message-ID: <6222@amc-gw.amc.com>
Date: 15 Mar 91 17:31:42 GMT
References: <1991Mar14.212645.10354@ssd.kodak.com>
Reply-To: jwbirdsa@europa.amc.com (James Birdsall)
Organization: Applied Microsystems, Redmond, WA
Lines: 54

In article <1991Mar14.212645.10354@ssd.kodak.com> staffan@phos.serum.kodak.com (Kenneth Staffan (x37507)) writes:
>I often use floppies to port stuff between my 3b1 at home and PC-AT at work.
>I always format the floppies at home.  When I write the disk at home & read
>it at work, it _always_ works.  I have a problem going the other way, though.
>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.

                      OLD_DATA_OLD_DATA_OLD_DATA_OLD_DATA  \
width of 1.2M head {  NEW_DATA_NEW_DATA_NEW_DATA_NEW_DATA  | width of
                      OLD_DATA_OLD_DATA_OLD_DATA_OLD_DATA  /    360K head

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
will be no old stray data to cause trouble. The low-density drive should
be able to read this disk, unless the field strength is extremely low, but
I've never seen that happen in practice.

   For transfers from the 3B1 to the PC/AT, use a disk formatted to 360K on
the low-density drive. This disk does not have to be virgin, since the
low-density head is wide enough to completely overwrite anything that was
there before. Write to this disk ONLY with the low-density drive, since
writing to it with the high-density drive will cause the read errors that
started this discussion in the first place.

   As long as you abide by the rules on what drive to use for writing,
either of the drives can be used to read any disk.

   This topic has been discussed for years in the MS-DOS world; the
preceding techniques have proven about as reliable and foolproof as
anything can be.

-- 
James W. Birdsall   WORK: jwbirdsa@amc.com   {uunet,uw-coco}!amc-gw!jwbirdsa
HOME: {uunet,uw-coco}!amc-gw!picarefy!jwbirdsa OTHER: 71261.1731@compuserve.com
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