My ubuntu laptop wouldn’t boot probably this evening as the /etc/mtab was corrupt. On boot up it wouldn’t fsck and repair the root file system and ubuntu doesn’t allow you to easily enter single user mode prior to mounting disks.
Fortunately I had a 6 month old copy of systemrescuecd in my laptop bag. This allowed me to boot up a miniature linux os from the cd and run fsck on the root file system. Eventhough the /etc/mtab was corrupt /etc/fstab was able to tell me the dev id for /.
It pays to keep one close by as you never know when you will need it….