My daughter has been using a 2T 2.5" external Seagate Disk. It has a USB3 interface, compatible with USB2. She uses it with her ThinkPad T520 which has only USB2 ports. A couple of weeks ago, the disk stopped working. The disk would power up but it would not show up. Oh oh, another disk failure. But it worked when I plugged it into my computer. In fact, it worked with any USB3 port and no USB2 port (even with a powered hub). I thought that this was impossible. So did Seagate support (I was talking with them on another matter -- two dead drives, one of which they had claimed was out of warranty when it was not). The support guy suggested trying a different cable, but neither of us thought that it was likely to work. Changing cables did work. So: a cable can fail in such a way that a USB3 device won't work with a USB2 port but will work with a USB3 port. The connector on the drive itself has a funny B shape. It is called a USB 3.0 micro-B socket. Apparently a USB 2.0 micro B plug can fit in half of it! Curiouser and Curiouser. I wonder if an ordinary micro USB 2.0 cable, as used with cell phones, would connect the drive to a computer's USB 2 port.