I don't think 12.5 is the correct answer for the capacity of the drive. 12.5gb is the portion of the drive BMW makes available for music storage. There is a bunch of other stuff on the drive (maps, firmware, etc) for the navigation system and likely for iDrive. Drives have never been commercially produced in 12.5gb (or slightly larger to account for formatting losses) capacities so it's extremely unlikely to be something of that size. Additionally it's pretty likely it is in fact an 80gb 2.5" drive as that's what would likely be both cheap but still commercially available.

Ben