After some test to determine how many Mp3 files on a USB drive you could keep in one folder that would be recognized to play on the S950 audio player is 499 (2.09 G) but its the amount of files in EACH folder not the overall data size.....if you put in any more files you will get an error message that says to many files in a folder. You can have as many folders as you wish depending on the USB size as long as you have no more then 499 in each folder. Hope this helps..