For my IT class, we were told to get an external hard drive and install Ubuntu 8.04 (8.10 has known issues with Samba Authentication). Since I am new to the Linux world (I can get around and program on CLI, just have had not much knowledge as far as installation and GRUB. The install was from the Ubuntu Live CD and I installed like normal but made sure to only format and install on my External Drive (Western Digital 320g). I included GRUB boot loader on install screen 7 under the Advanced options and made sure to install it to my external as well. When restarting my computer to boot from USB, it wouldn't load the GRUB menu from my external, it just went to my normal computer's boot process. When examining the Bios, I ensured that the boot order for my machine had USB as the first option in the list. What I failed to notice and would have saved me a lot of time in the beginning was that I didn't go into the Advanced Bios options and set the Bios to look for USB devices on boot. After this was enabled, I got all sorts of errors. Finally, when surfing the net for answers, I found Ubuntukids.com with an article saying that the external hard drive is acting as primary and so the Grub boot loader should look for it on hd(0,0) and not what it installed as (i.e. hd(1,0)). When editing the GRUB boot entries to look for hd(0,0), the external hard drive booted properly.
Please forgive my lack of formatting, I don't have much time to write this entry.