Article:
  Creating a Dual-Boot Windows XP and Ubuntu Laptop
Subject:   Same steps, but in USB external hard disk, what did you think?
Date:   2006-08-08 08:46:51
From:   kfarnham
Response to: Same steps, but in USB external hard disk, what did you think?

I think the question will be whether the USB hard disk can be made bootable. A bootloader may "see" a disk, but if it doesn't consider the disk a bootable device, then you won't be able to boot from it.