Ubuntu – /dev/mapper/vg00-root does not exist

I FIXED IT!! For the references of future generations, heres how:

The problem was that by plugging the a clone of the system I was running into it, I'd "confused" my system as to which drive to write to, and somehow nuked the Volume Group metadata for the original internal drive. Luckily, I still had the metadata from the clone I'd made.

This meant that I could plug in the working backup, unlock it, and use vgcfgbackup to make a copy of its metadata.

~$ sudo vgcfgbackup -f /tmp/trousers
  Volume group "ubuntu" successfully backed up.

This creates a text file with the metadata in /tmp called trousers (called it whatever you like, makes no difference)

Then, I dismounted and unplugged the functioning backup clone, and plugged in the borked internal drive. Unlocked it like I did the backup, then run:

 sudo vgcfgrestore -f /tmp/trousers ubuntu
  Restored volume group ubuntu

Upon doing this, the volume group suddenly sprang to life in disk utility, and the borked internal was unborked, mountable and readable again!

Critical bit here is that 'ubuntu' was the name of the original volume group. You can verify this by opening up the 'trousers' file in gedit and looking at the entry on the line below "creation_time".

Also worth noting that this only worked because the backup was an exact clone of the original internal, and hence the VG metatdata file had all the correct UUID's in place. I actually got an error message first time round because I'd originally manually changed one of the UUID's on the external drive. I had to use pvdisplay on the original internal to find its UUID, and then manually edit the 'trousers' file to get it to work.

Anyway, hope that helps someone one day.

Your most recent kernel, 3.5.0-47-generic, is missing its corresponding initrd image. Pick the previous kernel version from the boot menu and once the system comes up, run sudo update-initramfs -c -k 3.5.0-47-generic to create the initrd, and sudo update-grub.