try loading a CDR/DVD-R of mp3 files. Save a couple to the HDD. Then try resetting the RNS (< > & i [far right hand/top button/key])
When the RNS starts up again, check that you have the copied mp3 files on the HDD (view "MEDIA" ). If they are there, then try the map disc again.
Sometimes the HDD becomes inactive, and needs to be 'woken up' by forcing it save then play mp3 files.