But before you purchase, be clear on how you plan to use the drive. For basic file backup there are plenty of choices from Western Digital, Seagate and others. But if you plan to use the drive to edit video make sure the drive meets the demands of the application.
I've discovered that while it's true that the USB 3.0 interface can support transfer rates up to 5 Gbit/s, many of these drives fail to spin fast enough to support the demands of video editing. How fast are they? Well that's difficult to tell because the manufacturers often fail to provide that information. They also often omit the speed of the drive itself. My guess is that since these are bus-powered drives, they probably spin at around 5400 rpm - often not suitable for post production needs.
However, the one exception that I've found is the G-Technology G-Drive Mobile USB. G-Tech offers a 7200 RPM version which can also support Thunderbolt.
G-Drive Moble USB
Only one caveat with this G Drive. It's formatted for Mac so for Windows use you'll need to repartition and reformat the drive. Here's how.
And G-Tech deserves credit for actually including basic specifications missing from Seagate and Western Digital products (which is even weirder in that Western Digital and G-Tech are divisions of the same company).