Last Friday I promised a good friend that I will provide him with a LiveCD that contains a bunch of anti-virus software that will be able to scan his heavily infected windows machine. It continues to annoy me how Microsoft can continue to get away with utter and complete incompetency in shipping totally flawed operating system software. I downloaded PLoP live CD and went about setting up a bootable USB drive. Unfortunately, in that process, I accidentally messed up the master boot record of my functioning Fedora 11 machine. So, this post is really to remind me what to do to recover from these mistakes in the future.

a) Boot from a live CD – F11 would suffice
b) If the drive that needs rescuing is the internal drive (usually /dev/sda) and if the boot partition is /dev/sda1 (usually), then do the following:
i) Open up a Terminal
ii) Become root with the command (F11 root has not password):
$ su –
iii) run the grup command:
$ grub
iv) within the grub prompt do the following:

grub> root (hd0,0)
grup> setup (hd0)
grub> quit

Note that the line “root (hd0,0)” refers to the 1st drive on your system and the first partition. GRUB starting from 0. So hd0 is the first drive and 0 is the first partition.

Now, if you don’t know which one is the boot partition of your drive, then you will have to run fdisk /dev/sda. For example,

[root@qbic ~]# fdisk /dev/sda

The number of cylinders for this disk is set to 4998.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)

Command (m for help): p

Disk /dev/sda: 41.1 GB, 41110142976 bytes
255 heads, 63 sectors/track, 4998 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x6f1824f1

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          26      208813+  83  Linux
/dev/sda2              27        1501    11847937+  83  Linux
/dev/sda3            1757        4998    26041365   8e  Linux LVM
/dev/sda4            1502        1756     2048287+   5  Extended
/dev/sda5            1502        1756     2048256   82  Linux swap / Solaris

Partition table entries are not in disk order

Command (m for help): 

The output above is generated after invoking fdisk /dev/sda and hitting the p key. Naturally, you need to be root to do this. So, if the boot partition was say /dev/sda2, then the grub command above would have been root (hd0,1).