From other posts I've read the drive is a specific auto-motive grade drive for vibration performance etc, which in itself is not a problem, as replacing it with a completely solid state drive which are cheap now-a-days would be sufficient. However there was also information that the file-system was custom to the iDrive (the drive also contains all the iDrive operating software) so you'd need to carefully clone the data between the new and old device, then identify the partition reserved for music and try to resize that partition.
Tricky, easy to break something, probably not worth it.