Booting off an external harddrive

[Impurity]

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?

[Kotto*N[ix]mouth]

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.