On one of my machines I had an ubuntu installation on a single partition (sda3). I recently moved everything except /home onto another partition (sda6) and set up grub to boot from that partition.
This was all working fine until there was a kernel update. The update reconfigured grub to try boot from the original partition, which failed, but fortunately grub lets you edit stuff before booting and I was able to fix it without too much pain.
sudo update-grub
This will have been run when the kernel was updated, and will actually be what caused Glenn's original problem. Glenn, you'll need to edit /boot/grub/menu.lst in a text editor, and find the line that looks similar to this: # kopt=root=/dev/sda3 ro It may refer to a UUID, like so: # kopt=root=UUID=d8f53f29-5c11-4562-8e80-e33feebd98e9 ro Change this line to read # kopt=root=/dev/sda6 ro or whatever more closely matches your existing one, and then run "sudo update-grub". Check the resulting file to make sure it's retained this setting - if you look further down the file you'll find the actual grub configuration blocks, and they should reflect your change. It's worth stressing here that the lines I pasted above are commented for a reason. Do not uncomment them! These lines are how update-grub manages the configuration file - it uses a series of commented lines to automatically build up a config file based on the kernels it finds you have installed. If you read the existing comments carefully, in particular the bits with ## at the start of each line, it'll explain how the system works. And, if you want to just add a config stanza that you know is fine and will work, you can add it before the line that says "### BEGIN AUTOMAGIC KERNELS LIST" and update-grub will never touch it.