Imagine for any reason you have any other crappy 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.
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 Device Boot Start End Blocks Id System /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)
If you have grub-legacy 2 and your root partition is /dev/sda3 then you can re-install grub in the MBR of your hard drive by typing the following commands :
sudo grub grub>root (hd0,2) grub>setup (hd0) grub>quit exit
Note that in grub-legacy (hd0,2) is the third partition.
Then choose "Reboot the system", everything should be fine.