Description:  
The MMCs of the S7-300 may be read only by specific devices outside the CPU. See here Entry ID 21829579.

Warning:
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 ID 13008499.

Description:  
If an MMC of the S7-300 is used for non-SIMATIC purposes or is formatted in a third-party device, then the internal structure of the MMC is overwritten and cannot be restored. In this the SIMATIC CPU no longer recognizes/accepts the MMC. It is then no longer possible to use this MMC in a SIMATIC CPU.

Conclusion:
The MMCs of the S7-300 may be read or written only by specific devices outside the CPU. See here Entry ID 21829579.

Description:  
The MMC in the CPU requests an overall reset. This is indicated by the STOP LED blinking at 0.5 Hz. The reasons for this are the following:

• The MMC in the CPU is not formatted.
• The MMC contains data that cannot be interpreted by the CPU. The reasons for this can in turn be the following:
  - the MMC contains the configuration of a different CPU type.
  - the version of the CPU doesn't support the memory size of the MMC.
  - the MMC contains an operating system that is not compatible with the CPU.

Note:

• How to reset an MMC is described in Entry ID: 29801950.
• More information on handling the MMC is available in the manual "S7-300, CPU 31xC and CPU 31x: Installation Instructions" in Entry ID 13008499.

Instructions:
The error message concerned indicates that the MMC is no longer recognized/accepted by the SIMATIC CPU. This is the case when the SIMATIC IDs have been overwritten, for example through using the MMC in a non-SIMATIC application, or the MMC has been formatted in a third-party device. Unfortunately it is not possible to restore the internal structure of the MMC for renewed use in SIMATIC S7-300 CPUs.

See Entry ID 21829579 for which SIMATIC devices you can use to read from or write to S7-300 MMCs. More information on handling the MMC is available in the manual "S7-300, CPU 31xC and CPU 31x: Installation Instructions" in Entry ID 13008499.