I have an installation of Ubuntu 12.10 on a USB3 device, and it occasionally disappears during boot, dropping me into busybox. This is what I've been able to figure out so far:

During a single boot, the following happened:

1. At the grub2 menu, I typed c and dropped to the grub> prompt
2. I typed ls -l and got a list of all the devices, including partitions and UUIDs - the USB3 partitions were available
3. I escaped back to the boot select menu, selected the default item (ubuntu) and hit enter
4. The screen went black for a second before turning into the purple Ubuntu boot screen with the dots (which usually indicates that the boot will fail. When all is well I don't get the black screen)
5. The boot dropped into BusyBox v1.19.3 with the message `ALERT! /dev/disk/by-uuid/[uuid] does not exist
6. blkid displays all of the partitions except those of the USB3 device, as does ls /dev/disk/by-uuid or any of the alternatives.