I don't use the built-in hd for music, I find to be to much of hasstle to keep it updated. I sometimes connect the IPOD however 90% of the time I simply use a USB stick.
I find the USB stick is the easiest way to keep my tracks up to date, make playlists and you can also maximize the audio quality.
I know I didn't answer your question, however just thought I would share my experience.
2009 335i E92 Coupe
Alpine White/Coral Red Dakota