weird it should at least show up. i'd say you mount it in the terminal if nothing happens then it may be a disk failure. but of course if it works on other machines with windows or linux on them then it truly is a weird issue.
edit: did you try both ports? and did you try to plug it in with the mba charging?
Ok so my linux use has apearently spoiled me. You should see the disk in disk utilities if not then i think you should try it with a different computer. Ask you friend or something.
I think all USB ports on all PC's or Mac's are bus powered. Problem is that some external devices need more power than the USB can deliver and consequently do not work.