The MMCs of the S7-300 may be read only by specific devices
outside the CPU. See here Entry ID 21829579.
Do not use the MMC for non-SIMATIC
purposes and do not format it with a
third-party device. This would possibly overwrite the internal
structure of the MMC that cannot be restored. In this case the MMC
is no longer recognized/accepted in a SIMATIC CPU and can no longer
be used with SIMATIC devices.
More information on handling the MMC is available in the manual
"S7-300, CPU 31xC and CPU 31x: Installation Instructions" in Entry