Hi,
Obviously lots of discussion and I may have missed it but what format are the files that you have written to the USB stick? What are their file extensions?
As always - the USB connection is intended for small transitional collections of music (i.e. “I just bought this album, let me take it to my friends to let him hear it…”) rather than a permanent place to put your own collection - for that we would recommend that your music is stored on a proper UPnP server so that it can be browsed, sorted and filtered more flexibly than just the folder and file browsing offered by the USB input.
P