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From: tanida@forseti.css.gov (Tom Tanida)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Formatting a 1.44 MB disk to 720 kB...how?
Message-ID: <718@esosun.UUCP>
Date: 17 May 91 20:16:41 GMT
References: <1991May16.220514.23423@ariel.unm.edu> <1991May17.081931.7014@agate.berkeley.edu>
Sender: news@esosun.UUCP
Reply-To: tanida@esosun.css.gov (Tom Tanida)
Organization: Science Aplications Int'l Corp.
Lines: 40
In-reply-to: c164-al@katerina.uucp (Joon Song)

In article <1991May17.081931.7014@agate.berkeley.edu>, c164-al@katerina (Joon Song) writes:
>It seems that most 3.5" drives look for a hole in the disk casing to 
>determine the density of the media.  So if you want to format a high-density
>3.5" disk to 720K, just cover the high density hole with some scotch tape. 
>Now you have a high density disk which looks like a double density disk to
>the disk drive.  However, if you take scotch tape off and uncover the high
>density hole, the disk will probably be unreadable on most machines.
>
>Now some people out there are probably going to say something like:
>  "all you need to do is type 'format b: /t:80 /n:9' from the command line".
>
>Well, that may work on some machines (most notably IBM PS/2s), but on
>most machines, a high density 3.5" disk drive will refuse to format a HD
>disk to DD for the same reason that it will refuse to format a DD disk to 
>HD.  (I know that doesn't make any sense, but it's true.)

A lot of drives don't look for the extra hole that is characteristic
of the HD media, but, because of all the types of disk drives out
there, you never know (and hence I agree with this).

There is another issue:

You will get some kind of weird failure error if you take a previously
formatted, HD disk and try to low-density format it.  This comes from
the 'media descriptor' byte, located on the very first absolute sector on
the disk.  For some lame reason, some format programs look at this and balk
if you attempt to switch densities on them (including the Norton format
program, at least in version 4.5).  If you 1) use a magnet to wipe the
disk out, or 2) use some sector editor to change the media descriptor
byte, you can switch formats.  I don't know off hand what the values
are or what the exact location of this byte is on the disk (other than
that it's in the first absolute sector- sector 1, not 0, don't ask me
why that is either :-) ).

This puzzled me for the longest time until I stumbled across it in
Hayden Books' "Advanced MS-DOS Programming", which covers both
absolute and MSDOS logical disk layouts.

-Tom
tanida@esosun.css.gov