I bought a new DGX660 primarily to listen to audio files and lay down keyboard tracks on top of it. For some reason, the keyboard is not seeing the .M4A files on the USB thumb drive. I've reformatted it multiple times, renamed and recopied files, put them in root and user files directory and my only choice seems to be new audio. They play fine off the same media on my PC, but not the keyboard. One time the keyboard recognized one song file and played it back fine. Ever since though, i have been unable to add, listen to and play along with any other files on the same thumb drive. manual offers no help on this issue. Any idea what I'm missing? Or websites where i can get more info/tutorials on the keyboard?