See How do you reset a SIMATIC S7-300 CPU and MMC

MMC
It is only possible to reset the MMC if it contains data^* that is invalid for the CPU.

(^* Invalid system data is, for example, data stored for a different type of CPU.) Otherwise the MMC is only initialized by the procedure described.

All the user data is deleted when you reset the SIMATIC Micro Memory Card (MMC). Proceed as follows.

1. The MMC is slotted in the bay of the CPU. The CPU requests an overall reset (slow blinking of the STOP LED).
2. Set the switch to "MRES" and hold it there until the STOP LED lights continuously (approx. 9 seconds).
3. Within the next 3 seconds, you must let the switch go and set it again to "MRES". The STOP LED now blinks during the delete procedure.

If the CPU continues to request an overall reset after the procedure described above, then the MMC is defective.

Check (Which devices can you use to read from, write to or delete from an S7-300 MMC (Micro Memory Card) outside the CPU?) for list of devices which handle MMC.

regards