I would think that the first problem you would run into would be a possible device name difference. Most internal hard drives (IDE) are labelled as /dev/hda. However, a USB drive is recognized as a SCSI device, thus /dev/sda. If you were to check out your /etc/fstab file, it will probably be full of hda instead of sda.
Since your laptop does support booting off a USB device in the BIOS, I suppose this is probably one of those things you may be able to get working, but the effort may not be worth the reward.