Recover Grub

Imagine for any reason you have any other crapy OS installed next to Debian. And for any reason you need to reinstall it because is really broken. Again, imagine this crappy OS has a crappy installation and overwrites the disk MBR to own the full computer. What's next? reinstall Debian? no

In any case, you can use SuperGrub, a boot utility that can restore and repair overwritten and misconfigured GRUB installs or directly boot various operating systems.

Recover Debian/Etch

Standard procedure

The Debian Installer (DVD,netinst CD,floppy, netboot, etc.) can be used to rescue systems, for example if they fail to boot after an upgrade. You should read and follow the instruction "Recovering a Broken System" section of the Installation Guide1 (look for it in the index) [link to stable/i386].

Troubleshooting : 'grub-install /dev/sda' failed.

If D-I's "Reinstall GRUB boot loader" option fails, with error message

Unable to install GRUB in /dev/sda
Executing 'grub-install /dev/sda' failed.
This is a fatal error.

Also, the d-i log console (Alt-F4) displays grub-installer: /dev/scsi/host0/bus0/target0/lun0/disc does not have any corresponding BIOS Drive You can still use the following method to reinstall grub. Choose "Execute a shell in Your root partition" in the d-i rescue menu.

Locate your root partition : {{{fdisk -l

Disk /dev/sda: 39.9 GB, 39996820992 bytes 255 heads, 63 sectors/track, 4862 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes

/dev/sda1 * 1 111 891576 7 HPFS/NTFS /dev/sda2 167 230 514080 82 Linux swap /dev/sda3 231 784 4450005 83 Linux /dev/sda4 785 4862 32756535 f W95 Ext'd (LBA) /dev/sda5 785 4862 32756503+ 83 Linux }}} Reinstall grub bootloader (grub manual)

grub>root (hd0,3) //assuming your root partition is /dev/sda3
grub>setup (hd0)

choose "Reboot the system", everything should be fine.

Debian/Sarge and previous

To recover grub Debian/sarge, Debian/woody.. you need a console from a live CD. For example, boot with Debian installation CD and hit ctrl+alt+F2. Other live CDs will also work, such as Knoppix. Then mount the Debian partition on the hard disk, change the mount point to be the new root and exec grub-install.

May be these commands clarify the steps. For this example, Debian is in /dev/hda2

 # mkdir /my-debian
 # mount /dev/hda2 /my-debian
 # chroot /my-debian
 # grub-install /dev/hda
 # exit
 # umount /my-debian
 # reboot

You may need to load the fs module manually before mounting, for example:

 # modprobe reiserfs

That's all. No more, no less.