@igorsk: plugged into USB port of computer.
@bbriand: haven't yet tried a different cable - that's the first thing to try. I kinda doubt it'll help though, since once the connection stabilizes, moving the cable around doesn't interrupt it. Haven't tried a different computer yet either - good idea.
I installed all the software using my prs-600. do i need to reinstall using the mounted prs-650 drive, or should everything be ok as is?