What is the difference between USB 2.0 and USB 3.0? I was shopping for a Western Digital backup drive and then after purchase, noticed that it just said "USB 2.0". Then online I noticed that there is a USB 2.0/3.0 version. Should I return my purchase of the USB 2.0 version......and get a USB 2.0/3.0 version instead? Does it matter? Thanks!