I am having the same problem. I recently purchased a 8GB SanDisk Cruzer. I uninstalled U3, reformatted (FAT32), and loaded a bunch of mp3s on it. I plugged it into my F900BT and it worked. The system was slow, but it worked.
After playing around with it for a while, I decided to add folders to the drive. I plugged it back into the deck and no go. The USB icon on the menu no longer lights up. The light on the USB drive, however, is lit up and showing activity.
I have reformatted the drive, reset the deck, and cursed up a storm.
Anyone have any ideas? Thanks!
