I'd get the Seagate one that was mentioned earlier (they come in several sizes). USB 2.0 isn't as fast an interface as Firewire or Thunderbolt, but it's fine for your purposes (and, of course, cheaper). Before you use it, use Disk Utility to reformat it as an HFS+ drive. You might want to use it as a Time Machine drive too. In that case I would partition it to dedicate a partition to TM.