I am running Ubuntu 14.04. Using GParted I have copied the Ubuntu partition to an external HDD connected via USB.
The partition with Ubuntu on the external HDD has its flag set by GParted to "boot". There is one other partition on the external HDD which contains non-bootable Windows Xp data. The Grub bootloader correctly shows the operating systems on the external drive and also on a second internal HDD. The Ubuntu partition on the external HDD will not boot (I want to use this partition as a backup which I will periodically update).
It doesn't show the external USB drive at all - only the 2 internal drives.
There is no response to
sudo blkid -c /dev/null -o list