GNU GRUB (“GRUB” for short) is a boot loader package from the GNU Project. GRUB is the reference implementation of the Multiboot Specification, which allows a user to have several different operating systems on their computer at once, and to choose which one to run when the computer starts. GRUB can be used to select from different kernel images available on a particular operating system’s partitions, as well as to pass boot-time parameters to such kernels.
Every once in a while windows may have the inclination to overwrite your bootloader with its own. This will make accessing your system impossible.
Well, there is a solution to override the 512 Kb of your drive that represent the MBR.
In the fact you can follow these steps to repair your GRUB loader:
Boot your PC with a Live CD in the drive, after run fdisk to display list of your partitions system.
$ sudo /sbin/fdisk -l
Disk /dev/sda: 250.0 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0xa588e9cb
Device Boot Start End Blocks Id System
/dev/sda1 * 1 30071 241545276 83 Linux
/dev/sda2 30072 30401 2650725 5 Extended
/dev/sda5 30072 30401 2650693+ 82 Linux swap / Solaris
In that fdisk example, the root linux partition reside in /dev/sda1
Create a directory /mnt/root and mount the linux partition.
$ sudo mkdir /mnt/root
$ sudo mount /dev/sda1 /mnt/root
Change the root “/” directory to our old root directory the /mnt/root
$ sudo chroot /mnt/root /bin/bash
Execute the grub command
$ sudo grub –config-file=/boot/grub/menbu.lst
GNU GRUB version 0.97 (640K lower / 3072K upper memory)
[ Minimal BASH-like line editing is supported. For
the first word, TAB lists possible command
completions. Anywhere else TAB lists the possible
completions of a device/filename. ]
Now we need to tell grub where are the grub files:
If you don’t know where they are, type:
grub> find /boot/grub/stage1
Now type the following, replacing hd0 with the physical drive linux is installed
grub> root (hd0,0)
Remember that for grub (hd0,0) means hda (primary controller master), first partition.
Let’s install the grub on hd0
grub> setup (hd0,0)
$ sudo reboot
Now you can reboot safely, with your linux system.