News Group Posts

mbr master boot record partition layout

From: Dances With Crows <danceswithcrows@usa.net>
Subject: Re: MBR of pc(x86) ? depends on CHS geometry
Date: 27 Apr 2002 20:14:24 GMT
References: <aaeii0$m9n$1@ctb-nnrp1.saix.net> 

On 27 Apr 2002 16:10:40 GMT, cglur@onwe.co.za staggered into the Black
Sun and said:
>  when DOS fdisk \MBR executes does it write the MBR according to
> the geomentry of the specific disk ?

Actually, I think it'd give a message similar to "Invalid argument."
Perhaps you meant "FDISK /MBR"?

If you use the undocumented /MBR switch to FDISK.EXE, the first 446
bytes of the first sector on the disk are overwritten and replaced with
a "standard" DOS bootsector.  It doesn't overwrite the partition table
or otherwise fiddle with it.  This standard DOS bootsector examines the
partition table, finds the first primary partition that's marked active,
then loads the first sector from that partition and executes it.  Under
DOS and derived (Lose9x/ME) the first sector of the DOS partition
contains the first sector of IO.SYS.

> Or can I copy the MBR from another disk ?

Not the entire MBR, just the first 446 bytes.

> All 5 of my IDE drives have become non-DOS-bootable.
> Which is not a great problem, since I boot my prefered OS
> (oberon: native or DOSbased or LinuxBased) and Linux via fd0.

That's not a great idea.  Floppies die, and they're slow.

> When I tried to partition a friend's hda to take linux as well as his
> W95, again his hda became non-DOS-bootable !
> I used linux fdisk.  I need to fix his disk !!

Booting DOS and derived systems from LILO or GRUB is covered extremely
well in several HOWTOs.  If you've got a Linux/DOS dual-boot
system, the best thing to do is to have LILO or GRUB boot both OSes.
You can do this all from Linux.

> For some of the disks which I've investigated, DOS's fdisk doesn't
> even see the device although the BIOS auto detect sees the correct
> CHS, and linux or Oberon can read the various partitons & files.
> A DOS 'officinado' tells me that if DOS-fdisk can't see it, I can't 
> restore the MBR ?

Not with FDISK, you can't.  Fortunately, you can get a valid DOS MBR by
doing "dd if=/dev/hda of=/somewhere/dos.mbr.raw bs=446 count=1" if you
have a /dev/hda with a valid DOS MBR on it.  Reverse the if= and of=
arguments to put this valid DOS MBR anywhere.
 
> Thanks for any advice, also emailed to cglur@onwe.co.za

You mailed a copy of this Usenet post to yourself?  Or are you trying to
say, "Please CC: me a copy of any replies,"?  If it's the latter,
there's a time-honored tradition in Usenet that goes, "Post here, Read
here."  If your NNTP feed is flaky, then
http://groups.google.com/advanced_group_search is very nice for tracking
down posts you made and any replies.

-- 
Matt G|There is no Darkness in Eternity/But only Light too dim for us to see
Brainbench MVP for Linux Admin / 
http://www.brainbench.com     /  "He is a rhythmic movement of the
-----------------------------/    penguins, is Tux." --MegaHAL
 

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