Fixing a broken grub MBR with LVM / and /usr

Following a power cut, a friend of mine found that he couldn't boot his Debian etch machine. It appeared that some traces of lilo were still lurking on the machine and had messed up the MBR during a kernel upgrade. As the partitions with the exception of /boot were in LVM volumes, it made the task of recovering the MBR a little more involved.

Here is the procedure we followed:

Boot from a Debian install CD into rescue mode and after following all the prompts get a shell on the system root
Copy the current fstab to fstab.safe (we need this to get the file system back in order later)
Re-boot from the Debian install CD, this time going through the normal route.
At the partition disks prompt, choose Manual.
Then choose the "Configure Logical Volume Manager" option.
Choose to keep the current config and activate the current volume group.
Pick finished to go back to the partitioner.
In the list of partitions, find the root and usr partitions and select each of them in turn option to use them as ext3 file systems with the appropriate mount point.
Choose the write changes to disk option. The installer should say it is going to format the swap partition and nothing else. If it says it is going to format anything else then go back and check how you have configured that partition.
When it brings up the message about an unclean system, use Alt-F2 to switch to the second console
Press enter to access the shell and type "chroot /target"
You are now in your original file system.
Mount the boot file system (probably with either mount /dev/sda1 /boot or mount /dev/hda1 /boot depending on your disks)
Now you can re-initialise grub with grub-install /dev/sda1 (or /dev/hda1) and use update-grub to ensure the menus are built.
Finally cd into /etc and copy your safe copy of fstab back over the one which the installer will have written out for you.
Then Ctrl-Alt-Del and you should be able to boot happily back into your original system.