Limits you down to truecrypt or maybe bestcrypt. Truecrypt being a bit up in the air about whether it is "safe" or not. - if your simply looking for something to keep your average person out, then i suppose this doesnt matter so much.
if you can remove the door mat requirement (macs) could go with a bitlocker password protected VHD on the external drive.
Any comments made are personal opinion and do not reflect directly on the position my current or past employers may have.