Wow - this describes the exact same problem I just started having about a week ago with my Rio player. It started out where it would be playing a song and suddenly start stuttering or pausing between songs. At first, I thought I had some improperly encoded MP3 files on the drive that needed to be deleted and redone. But then I got the "no hard disk found" message one morning. Usually, by pulling it out and re-inserting it in the car, it would start worknig again for a while - but eventually get the error again.

The frustrating thing is, every time I used it in the house, it worked perfectly! Just now, it finally started acting up while synching to my PC though, so I'm off to open it up and see if I can find a loose cable. (At least I now know it's not a problem with wiring behind my dash.)