I haven't used GB-PVR to record to a USB drive, but I've used other pvr's (MCE, Sage) without problems. I would imagine GB-PVR would work OK too. Now, that's a USB2 link...older USB might not work.
USB 2.0 drives should work fine. The thoroughput of USB 2.0 is more than enough for streaming even the largest of bit rates of MPEG2 as per with USB 2.0 you can have upto 60 megabyte per second (480 megabit per second) transfer rate.