I've constantly had issues transferring music to this player and I think I finally figured out the reason why. I have all my music on an external Hard Drive connecting to my laptop via USB. When I plug the MP3 player in via USB it conflicts with my other USB devices. Sometimes my keyboard wouldn't work (also plugged in via USB through hub). I was able to navigate around inside folder system on MP3, but as soon as I copied from External to mp3. Disconnected.
Once I copied the music to my local hard drive and then transfered to MP3 it worked. Very flaky and lucky I discovered that.