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From: greiner@oahu.cs.ucla.edu (John Greiner)
Newsgroups: comp.sys.ibm.pc
Subject: Re: FAT problems
Summary: Media Descriptor Byte was corrupted
Message-ID: <28310@shemp.CS.UCLA.EDU>
Date: 19 Oct 89 23:37:53 GMT
References: <1989Oct15.220849.21435@uokmax.ecn.uoknor.edu>
Sender: news@CS.UCLA.EDU
Reply-To: greiner@oahu.UUCP (John Greiner)
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Organization: UCLA Computer Science Department
Lines: 32

In article <1989Oct15.220849.21435@uokmax.ecn.uoknor.edu> gejohann@uokmax.UUCP (Gene Edward Johannsen) writes:
>
>I've got problems.  Recently my FAT has become _slightly_ messed up.  It's
>not because of a virus or anything. Its just one of those things that happen
>now and then.  The error causes CHKDSK to think that my C: drive is not a DOS
>disk and the Norton Utilities doesn't think my C: drive is a hard disk, except
>for the DISK TEST program that doesn't see anything wrong at all.

     I ran into this problem myself a while back and spent a lot of
time tracking down the problem (and learned alot about hard disks
on the way).  According to Microsoft, the first byte in each copy of the
FAT table is a Media Descriptor byte.  This byte is used to identify the
type drive to DOS.  Though only the first FAT table is usually accessed
for this info, even there it is rarely used.  Apparently, there is another
copy of this value in the boot sector (or is it the partition table?).  Some
programs look at one, other programs the other.  If one value gets corrupted
to a value that doesn't have a meaning defined by Microsoft, a program may
consider the disk to be a non-DOS disk (as chkdsk does).  Other programs 
which are only looking to see if the value corresponds to the value(s)
assigned to hard disk formats, may, upon detecting a corrupted value,
declare the drive not to be a hard drive.  Programs which look to the other
location for the Media Descriptor byte will run just fine.  Microsoft 
gave me this information (as well as the location of the Media Descriptor
byte in the boot sector/partition table, but I don't have the letter with
me...).  If you have access to Norton Utilities, you can check the first
byte of the 1st FAT table (make sure to use hex mode, not the fat table
editor as it doesn't show the Media Descriptor byte).  It should be F8 for
most hard drives, but I can't be certain that's what it should be for yours.
In my case, I changed it to F8, and the symptoms you described dissappeared.

   -John Greiner