Greg,
When ever I've had a problem I written to Ed Hamrick @ Vuescan....follow the links on his website as to what to include in your "help" message. He has never failed to write back quickly. He's a one one!
cheers,
Sam
I have had this on my 9000 and Vuescan in which case I eject the tray and close Vuescan. The problem disappears when I relaunch VS. I suspect it may have something to do with the scanner not having been fully initialised before VS is run. I remember reading somewhere that one should let the scanner start fully (green light stops blinking, takes a while actually), then insert the tray. And only then launch VS.