I've found that the hard drive is just too much of a PITA to be bothered with. It doesn't display album art, and there is no easy way to delete or add files to it without a complete wipe and re-import.
Once I discovered this I just bought one of the tiny USB drives and plugged it into the centre console socket - you can hardly even see it. Oh look, album art available and it's dead easy to add/delete music, change album art, MP3 tags, create play lists, etc. Job done.
Why BMW made such a pig's ear of the hard drive media software is a mystery to me.