I just had an experience where I was running an external hard drive on one of my two USB ports on my MBP. On the other USB port I had hooked in a 4 port USB hub without an external power source. On that hub I was running a mouse and a Nostromo Game Pad. When I started using this setup up, I started experiencing hard freezes while playing Skyrim. I knew the issue was most likely that the single port could not handle these two devices without an external power source on the hub. I moved my Windows Steam folder off the external drive onto my C drive, then I was able to ditch the external drive when running a Steam game. The Nostromo plugged directly into one of the USB ports, the mouse into the other USB port. Issue solved, FYI.