If you installed from a USB live media, you are probably experiencing bug 384633, where the initial creation of the grub boot config file uses a wrong device instead of a UUID. If you can boot off the live media, you can reinstall grub to the external usb disk, and that should fix the grub config file. You can also edit the grub boot commands (commands on screen to edit and boot) to fix the wrong devices. For the last few years, the fix was to simply reduce the disk letter or number by one, and upon successful boot, immediately run
With 12.10, the wrong device has changed, it may even be sda (which is usualy the internal hard disk). In 12.10, I have started seeing the disk lettering change, with the internal hard disk sometimes sdb instead of sda -- not even consistent. But after the update-grub, UUIDs will be used, and the disk letter will no longer matter. Feel free to add yourself to the bug, and maybe someday it will be fixed.