Siemens Simatic S7-300 Technical Data Manual page 78

Memory concept
4.2 Memory functions
4-16
Note
As a precaution against loss of data, always make sure that you do not exceed the
maximum number of delete/write operations. Also refer to the SIMATIC Micro Memory
Card (MMC) section in the "Structure and Communication Connections of a CPU"
chapter.
Caution
Data on a SIMATIC Micro Memory Card can be corrupted if you remove the card while it
is being accessed by a write operation. In this case, you may have to delete the MMC on
your PG, or format the card in the CPU. Never remove an MMC in RUN mode. Always
remove it when power is off, or when the CPU is in STOP state, and when the PG is not
a writing to the card. When the CPU is in STOP mode and you cannot not determine
whether or not a PG is writing to the card (e.g. load/delete block), disconnect the
communication lines.
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05