The minidisc assembly contains the solenoid which activates the cover flap on the front panel which covers the hole into which a disc is inserted. Ensure that the lever is re-engaged correctly when reassembling. Take special care when removing and replacing the screws which hold the minidisc drive mechanism onto the larger minidisc assembly. When removing the minidisc drive mechanism from the assembly, take care to avoid damage to the overwrite head. The best way I found to effect the removal was to pull the rear of the mechanism up first, which allowed the rest of the drive to be manipulated clear of the assembly.
Replacement is the reverse.
If anyone has anything further to add to this collection of tips, please let me know. In particular I would really like to obtain a copy of the service manual for the MDS-S30, or even just a schematic diagram. Note that this information is being provided as a service to the minidisc users of the world. Decks such as the MDS-S30 are mains operated and therefore have mains voltages present within the case when in operation. The information on this page is supplied without warrantee and with no guarantee.