The 20 MB harddisc is split into four partitions, with drive names D, E, F, and G (drive name C isn't used here, but may be used as RAM disc, in combination with Dobbertin RAM expansion).
Under [[FutureOS]] the four partitions are named I, J, K and L. They can be accessed at the same time, so all 20 MB are usable at the same time.
The hard disc is controlled by this I/O Ports: