The only thing with using the SD cards on the RNS-E is that you loose your playlists from (if you're like me) you have spent years assembling. I tried the dension Ipod connector but it's not for me (hissing and problems with BT).
I found this link on the web, others may have found it already. It gives you a script that when you run using apple script editor copies all the songs from a Itunes playlist to your SD card and creates a playlist. It converts all songs to MP3 and creates sub folders to arrange the music on the SD card.
Very useful! Saves manually copying files and creating and deleting MP3 versions of M4a files.
I've only got it to work in part so far. I can copy all the songs to the card but can't seem to locate the playlist file when browsing on the RNS-E.
Give it a go if you haven't tried already!