Experimentation is fun. After all, one of the things that makes Linux so interesting to a number of people is the ability to twiddle settings and see what happens - I'll admit that it's a major factor for me. One of the problems with that, though, is that some types of twiddling can lead to serious problems. A bit like sawing off the branch you're sitting on, in fact...
A number of people write to the Answer Gang with a query that goes something like this:
"Dear TAG: I have a stick of dynamite strapped to the CPU, and I'm not afraid to use it. Now that I have your undivided attention: I ran into a problem while trying to reinstall..."
What it turns out to be - after the police, the fire department, and the burly men in the white coats have come and gone - is that they've run into the classic "fried MBR" problem: install Linux, realize that Windows will screw up the boot record, delete the Linux partition, try to install Windows first... OOPS. The Windows setup runs into a problem and stops.
The reason for all of the above is that they forgot to uninstall LILO, which would have written out the original MBR; as it is, the boot code in the MBR is trying to pass control to the Linux kernel - and that's no longer there.
Nothing helps. The undocumented "fdisk/mbr" option that is supposed to write a clean Master Boot Record seems to have no effect; "fdisk" in interactive mode refuses to delete the "Non-DOS" partition; even the detonator fails to explode. What to do, what to do...
By the way, a factor in the first two problems might be the Windows "lock"
command - by default, 'raw writes' to disk are disallowed, and "lock c:" 'locks'
the drive to allow writing to it. (For the last problem, stick to the
bridge-wire type detonators from Dynamit Nobel, and store them
properly. :)
* * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * *
Note: The following advice will completely
wipe your Master Boot Record,
which contains all your partition information. DO NOT
DO THIS unless you
know that this is exactly the result you want - it
will leave your HD in
an unbootable state, in effect bringing it back to "factory-fresh",
i.e.,
empty of data and requiring partitioning and
formatting.
* * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * *
Linux-based solution
If you can still somehow fire up Linux - say, via Tom's Root-Boot floppy - you can simply invoke "dd", like so:
dd if=/dev/zero of=/dev/hda bs=512 count=1
Yep, that's it. That MBR is gone. Obviously, you have to be root to
do this.
DOS-based solution
Boot with a DOS floppy that has "debug" on it; run "debug". At the '-' prompt, "block-fill" a 512-byte chunk of memory with zeroes:
f 9000:0 200 0
Start assembly mode with the 'a' command, and enter the following code:
mov dx,9000
mov es,dx
xor bx,bx
mov cx,0001
mov dx,0080
mov ax,0301
int 13
int 20
Press <Enter> to exit assembly mode, take a deep breath - and press "g" to execute, then "q" to quit "debug". Your HD is now in a virgin state, and ready for partitioning and installation.
Obviously, you have to be root... oh, oops. Anybody that walks up with a DOS floppy can do this to your system in about a minute, including boot time. Let's see; where was that article about securing your box, again?...
The "dd" man page.
DOS-based fix: Original idea and code by Mark Minasi, used for clearing infected/damaged MBRs in a course of his that I used to teach; all code/command modifications mine.
Part of this document is based on informations from
rwcooper@mb.sympatico.ca
at
http://www.aplawrence.com/Bofcusm/481.html
I have noticed a lot of questions regarding recovering lost / damaged Master Boot Records (MBR) lately. So I thought it was about time to repost this.
You should make a backup copy of your MBR on a bootable floppy disk before
installing Linux and then again after installing Linux.
I can suggest two ways that you can back up the Master Boot Record (MBR) on
an IDE drive under Windows and DOS.
1. Use the Norton Utilities.
2. Use Debug, as found in DOS, it is also available from the DOS prompt
in Windows/95 so I assume it is also available in Windows/98.
Enter the following commands to save the MBR on the C drive, ignore
the text after the '<=' on each line as it is only a
comment:
| DEBUG MBR.DAT | Ignore the FILE NOT FOUND message | |
| A | Assemble a program | |
| MOV DX,9000 | Use segment 9000 | |
| MOV ES,DX | Setup the segment register | |
| XOR BX,BX | Clear BX to zero | |
| MOV CX,0001 | Start at track 00 sector 01, the MBR | |
| MOV DX,0080 | 80=C:, 81=D:, 00=A:, 01=B: | |
| MOV AX,0201 | Read 1 sector, 02=read | |
| INT 13 | BIOS disk i/o call | |
| INT 20 | Return to o/s | |
| Press the return key to end program entry | ||
| G | Execute the program | |
| R CX | Display the value of CX | |
| :200 | Change the value of CX to decimal 512, size of MBR | |
| W 9000:00 | Write the sector stored at address 9000 to MBR.DAT | |
| Q | exit DEBUG |
If you examine the contents of MBR.DAT using a disk file editor the last 2 bytes
must be AA55. At this point you should copy the MBR to a bootable floppy along
with DEBUG.EXE
This technique may be used to recover the MBR as well, assuming you can boot
from another device (say a floppy with DEBUG.EXE on it).
Enter the following commands to restore the MBR on the C drive, ignore the text
after the '<=' on each line as it is only a comment:
| DEBUG MBR.DAT | The file containing the desired MBR,
if you get a FILE NOT FOUND message type Q immediately! If you continue you will write garbage over the MBR. |
|
| L 9000:00 | Load the MBR into memory at this address | |
| A | Assemble a program | |
| MOV DX,9000 | The segment address containing the MBR | |
| MOV ES,DX | Setup the segment address | |
| XOR BX,BX | ||
| MOV CX,0001 | Track 00, sector 01 | |
| MOV DX,0080 | 80=C: | |
| MOV AX,0301 | Write one sector, 03=write | |
| INT 13 | ||
| INT 20 | ||
| Press the enter key to stop program entry | ||
| G | Execute the program | |
| Q | Exit DEBUG |
The MBR should now be restored to the C drive, making it bootable. For more
information on this technique for saving and restoring an MBR refer you
the book 'The Complete PC Upgrade and Maintenance Guide' by Mark Minasi,
published by Sybex.
Once you have Linux running you can save the boot record with the command:
dd if=/dev/hda of=/boot/boot.MBR bs=512
count=1
It can then be restored with:
dd if=/boot/boot.MBR of=/dev/hda bs=512
count=1
or if you do not want/need to overwrite the partition table with:
dd if=/boot/boot.MBR of=/dev/hda bs=446
count=1
as the partition table is kept in the last 66 bytes of the MBR.
DISCLAIMER:
Although I have double checked the above, I cannot be held
responsible for any errors. I suggest you try it on a bootable floppy disk
before using it on a hard drive. If it does not work on a floppy disk let me
know.
BTW: I find it easier to boot Linux from a floppy disk or CD-ROM than to recover
an MBR from DOS. The Slackware 3.5 (or greater) CD-ROM makes a good rescue disk
if you have a bios that supports bootable CDs.
#!/bin/sh
case $1 in
--read)
dd if=/dev/hda of=hda.mbr bs=512
count=1
;;
--write)
dd if=hda.mbr of=/dev/hda bs=512
count=1
;;
*)
echo "usage: MasterBootRecord.sh [
--read | -write ]"
;;
esac
exit 0
(Please do not copy and paste this text - it might fail - just type in the few lines..)
from http://www.unixboard.de/vb3/archive/index.php/t-4320.html
[ top ]