Unless you have some specific reason for using USB backup, consider a drive bay solution.
I've been using a StarTech HSD100SATBK Trayless SATA Drive Bay.
It's a cheap and fast direct SATA solution that allows using any SATA drive of any capacity as a removable-drive, completely eliminating USB, eSATA or other externally-cabled solutions and provides true SATA throughput speed.
I use it with WD and Seagate 250GB drives as removable backup devices.
Cheap, easy and fast. Hope this helps!