Once your USB has been been "registered" in the inTouch system, I believe it downloads all the track information, lists, and files to some sort of hard drive. I have duplicated this several times, where if I remove the USB drive and take it inside to my computer to delete file. When I bring that USB drive back to the Q50, it still thinks those deleted files are on the USB drive, AND is still able to play them. What it does is it moves the entire deleted folder to the bottom of your folder list, but it is definitely still there and playable. I've done a reset and they're still there. Also, what is frustrating is I added new folders to the USB disk, but they are now also at the very bottom under all the deleted folders. I also have a bunch of weird folder names like "@#%^@##^" that are not present on my USB drive on the computer.