Bootloader produces garbage - reconstructing MBR

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To: debian-user@lists.debian.org
Date: Sun,  3 Jul 2005 22:28:27 -0400 (EDT)

[Sorry if some of you are reading this message a second time. I was
not gated to the list properly, and this is a resend to the list
itself rather than newsgroup.]

Booting a scsi hd results in garbage after the scsi adapter's device
scan.

I have three scsi hard disks: sda, sdb, and sdc. My grub bootloader is
in sda MBR, and it is used to boot sda (disk not otherwise used), sdc
(my running system) or sdb (an emergency/utility disk).

It happened that I could no longer mount my ide cdrom or my scsi cdrom
drives, and in investigating this (no ide1 reported in dmesg) I found
that I could no longer boot using the bootloader in sda MBR:
Immediately after my Adaptec adapter scans the scsi devices, the boot
process hangs with the display of garbage characters.

If I boot each disk by itself with all other scsi devices
disconnected, I boot to garbage with sda, but sdb and sdc boot to a
blinking cursor after the scsi device scan. My sdb lilo loader has not
been used for a couple years, and my sdc grub loader has never been
tested, and I assume the blinking cursor is only a boot loader
configuration issue of lesser concern right now and unrelated to
difficulty getting the sda bootloader to work.

I can boot all three disks from a grub boot floppy, which I assume
narrows things down to what's in the sda MBR rather than a hardware
problem.

My current return for dmesg does not show a hardware problem either as
far as I can make out. I have put it on line at:
http//www.hartford-hwp.com/sandbox/dmesg.html

I copied (dd) the MBR on sda and sdc to img files. Being binary files,
I could not get diff to tell me anything more than that they differed,
but they are the same size (is any of this meaningful?). I couldn't
see any difference between when viewing them with beav. Although the
two hard disks are very similar (both are Hitachi/IBM, 36Gb, U160, but
sda is 10k rpm and sdc is 15k rpm). The operating systems are both
sarge, but use different kernels. I worry that I was not booting the
system I thought I was, and it is possible the two img files are the
same sdc file. But as I've described the situation, are not the two
files likely to be identical?

I need to do a MBR check, repair or restoration. Is there any
alternative (for a person of limited expertise) but to wipe it out and
start afresh? If there's evidence of corruption, which I assume the
gargage implies, is a wipe-out the only way to proceed?

To wipe the MBR clean, I read of doing # fdisk /MBR. However, there's
no reference to that option in man fdisk. Is it a good way to wipe out
the MBR so that it can be reconstructed? Other than making the disk
unbootable from its bootloader, which is already the case, are there
any particular dangers doing this?

Or should I simply overwrite the MBR with a new install of grub, which
I assume simply means doing >grub root and >grub setup.

-- 
 
       Haines Brown
       KB1GRM       
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