lane@cs.dal.ca (John Wright/Dr. Pat Lane) (02/08/89)
Hi. I've been experimenting with partitioning on an otherwise unused 40M
hard disk and I've run into a few things I don't understand.
I have tried several times to take a disk with two partitions and make the
second partition the active DOS partition. It doesn't seem to work. In
one case, for example, I was using MSDOS 3.21, starting with a 32Meg active
DOS partition followed by an 8Meg partition that was accessed by a special
device driver. I wanted to make the second partition a DOS partition, I
didn't care whether active or not, and I wasn't too interested in what
happened to the first partition for the purposes of the experiment (don't
ask me why...just for the hell of it!). My first approach was to diddle
the partition table with a program like Norton's Utilities Disk Editor,
zero out the entry for the first partition and change the entry for the
second one to show it as a DOS-12. Didn't work. After rebooting, DOS
didn't recognise the drive. Changing the position of the entry in the
partition table (1,2,3 or 4) didn't matter; changing the contents of the
existing boot sector of the second partition didn't seem to make any
difference; putting the first partition back in as a non-DOS partition
didn't help. Does DOS insist on owning the physically first partition
on the drive? Is DOS so fussy about things in the partition table and
in the boot sector being consistent that it just doesn't recognise the
drive if something doesnt match up...and what did I miss?
I tried using FDISK, deleting all existing partitions and creating a new
one at the appropriate cylinder. Again, DOS didn't recognise the drive,
even to format it. I don't understand why FDISK will let you create
a DOS partition anywhere on the drive but then DOS won't recognise it
unless it directly follows the partition table (or so it seems).
On the other hand I've noticed that depending on the partitioning
software, the first partition may start on the second absolute sector
or on the first sector of the second side of the first cylindar (17th
absolute sector).
I did notice that after re-creating a partition with FDSIK, several
sectors at the beginning of the created partition had been zeroed...
I hadn't previously realized that FDISK altered any part of the disk
other than the partition table. I take it that it's zeroing the part
to be occupied by the boot sector, FAT, and directory (though I don't
understand why, since FORMAT would take care of this and I don't know
how other O/S's use these areas). This feature could be a nasty
surprise (see below).
I tried much the same thing with PCDOS 3.30, 2 20Meg partitions, the
first an active DOS partition, the second, a DOS extended partition.
I put DOS on the second partition with SYS and I wanted to make the
second partition active and the first inactive. No way. FDISK would
not let me make an extended partition active; I could not do it by
changing the partition table with a Disk Editor; I could not do it
by deleting both partitions and putting them back on in reverse order
with FDISK (can't remember if I fixed the boot sector's etc. but it
wouldn't recognise the drive so I doubt I could have formatted it).
At some point I tried booting DOS 4.0 from diskette and running it's
FDISK...I'm not sure if this is when it happened but I wound up with
random garbage written into a good part of the first part of the first
partition...actually it was a few sectors seeminly repeated at random
points...some spots would be unaffected while others further on, mostly
on side 0 were overwritten. The second partition was similarly affected
though not as much. Sure trashed that disk (luckly, nothing important on
it). I realize that DOS 4 is quite a wierd beast but this seems a little
unusual. Mind, you I was a bit confused by this point so I'm not sure it
was DOS 4's fault.
I realize this is a long, confused, and sad story but I feel I'm missing
something fundamental about how DOS and/or BIOS deals with the hard disk
partition table. I'm hoping some kind sole can enlighten me.
BTW, some of the above experiences were with a Priam drive that used the
EVDR.SYS(?) deveice driver to access the second partition. I gather this
is a very strange disk system (In a separate posting, I ask how wierd is it?)
and that some or all of my problems could be explained (or at least dis-
counted thusly). However, I've had similar experiences with other drives
that used things like OnTrack's DMDRVR.BIN driver, as well as with DOS 3.3
which supports more than one DOS partition.
Thank you for very much for reading all this.
--
John Wright ////////////////// Phone: 902-424-3805 or 902-424-6527
Post: c/o Dr Pat Lane, Biology Dept, Dalhousie U, Halifax N.S., CANADA B3H-4H8
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