Another vote for Seagate.
I have a GoFlex, but the regular desktop version. It will say on their forums it won't work for Mac bla bla bla. In 3 seconds (or maybe 2) I got it running on OS X so maybe I'm a wizard
So no need to pay the extra 30 or 40 bucks for their "Mac Version" or whatever it's called.
Also I'm using that drive along a Western Digital (Green one?), but I don't trust WD anymore as I got 2 of them fail on me last year.
But to be fair, all harddrives can fail no matter what so make backups, bla bla (you know the drill).
So basically I'm telling you Seagate makes me wet, just because I'm on a personal vendetta against WD, but there are people doing it
au contraire.
Oh and last but not least those two drives (WD and Seagate) are the cheapest you can get nowadays. I personally distrust other companies for nothing more than "Ohhh they seem chinese" reason, but that's as cheap as it gets. I haven't looked up prices as of lately but I'm guessing the 3TB USB run for about 150 nowadays (they used to be 100, but blame mother nature and the asian floods for that).
I have the GoFlex 3 TB BTW, and also the WD is a 3 TB one.