I was having the same problem with books that were downloaded to Audible Manager prior to reformatting the Clip. Every time I re-formatted, none of the books that were already downloaded would play and I would get the “synchronize to listen to your audio books” message. Also, whenever I deleted books, the book was removed from the player but the memory did not clear either in the player or Audible Manager. So, even though there was nothing on it, the player showed that it was full. Then on a whim, I changed the USB mode to MSC in the Clip settings menu and everything worked correctly. All books are playable and the memory resets when books are deleted. I’m using Win XP sp2 with latest Audible software and Clip Firmware. Hope this works for you.