is it ok to have an mp3 player (zen touch in this case) plugged into the wall charger while transferring files? I noticed while transffering 2,000 files that the zen touch's battery ran out after just a couple hours, so i then decided to have it plugged into the wal mount to finish the transffers. I then found that after just around 20 min. or so, the unit became VERY warm. Should i do this, or just re charge the unit before finishing transfering the rest of my files?