Might be sticking.
I take it you're getting to the disk screen on the PB?
Try powering it on and listen out for any drive activity. If you hear some noises coming from it, it might be worth (if there's enough room) seeing if it splins up after jolt from your wrist.
It may actually be safer to open the drive up the lid and manually spin it (not touching the platters) to see if the motors pick it up. I've got a full size drive that does this.
As I did, only do this when you've eliminated other possible issues first.