I've been running 10.04 on an external USB HDD without problems for years.
I installed 12.04 on an external USB HDD and it worked until I upgraded to all the new packages. But after that it wouldn't boot. I just got into the grub prompt.
So then I tried installing on the disk inside the computer and that worked fine. But when I take it out and use it as an external USB HDD I just get:
error: hd0 out of disk. grub rescue>
How can I fix this? So why do I want this you might wonder? Well I have a laptop at home and a somewhat similar at work. And I just move the disk between them and don't have to move a heavy laptop. So I really hope I can continue doing this with 12.04!
The old disk is only 80Gb but the new one is 320Gb (7200rpm Toshiba). I'm using 12.04 on a 64Gb SSD as an external drive and it warns about out of disk but it boots anyway most of the times.