I'm looking to boot Linux off of an external harddrive. Is there anything I should know before doing this? Any suggestions?
Also, does anyone have any experience with Arch Linux? How's it compare to Ubuntu?
Yes. If you have your internal HDD connected at the same time, Linux will probably by default try to write Grub/Lilo to the MBR of /dev/sda, you will want to make sure that everything is written to /dev/sdb. I actually would suggest unplugging the internal drive just to be 100% sure no accidents happen.
I like Arch pretty well, honestly more than Ubuntu.
HERE is a three-part video guide on Arch, to help you out with it. Also, don't forget the
Arch Wiki.