First time asker, relatively new to Ubuntu/Linux. Haven't had much luck with the search function. Please forgive the essay below – the problem is a bit unique, I think…
I'm running 12.04 on a laptop (with UEFI BIOS), with full disk encryption set up up using the Alternate CD. I recently made a backup image of the entire hard drive using dd, to an external USB hdd. Having recently made some changes on the laptop, I tried to plug in the external USB clone to copy the files to it. Despite getting the password prompt and having it appear in the Disk Utility as a logical volume group, it would not actually mount the external drive, saying it was 'not a mountable file system'. Tried it on my desktop, and it mounted up just fine (once I installed lvm2).
Using pvdisplay, I noticed that both the external drive and internal laptop drive had the same UUID (duh, it's a clone!). So, on the desktop, I used pvchange -u to change the external drive's uuid. Plugged it into the laptop, alas, still no joy. Gave up, turned off the laptop (drive still plugged in), went and had dinner. Came back, unplugged external drive from now-off laptop, tried to boot. Problems!
It boots to the password prompt screen fine. Enter the password, and after a long wait, it drops to an initramfs prompt, with the error:
"ALERT! /dev/mapper/ubuntu-root does not exist."
If I reboot, and plug back in the external drive, it boots up, seemingly running root off the USB drive. I have tried the solution here, entering /dev/sda3 (and yet more variants) instead. No bingo, still get exactly the same messages. The fact I'm using LUKS with a LVM seems to complicate things. I think I've muddled a config file somewhere, probably by plugging in two drives with the same UUID (stupid!) and now it thinks /root is on the external drive. I'm stumped as to how to get it back.