Best of CUSM: linux save restore mbr master boot record lilo
With so many easily availible bootable rescue cd's today, I can't imagine this is of much value, but I leave it here just in case and perhaps as a reminder of the things we had to do way back when..
Message-ID: <399599F3.EE06B673@mb.sympatico.ca> From: Randy Cooper <firstname.lastname@example.org> Newsgroups: comp.os.linux.setup,comp.os.linux.misc Subject: Saving and Restoring an MBR Date: Sat, 12 Aug 2000 13:39:47 -0500 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 I 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.
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