By changing your MMC form one phone to another and when MMC was attracted by magetic fields condensations, and roughly handling the MMC , and more heat to the MMC and when it was used by various electronic items like Digital Camera, laptop, and memory card reader this file system will be corupted. And also when the mmc was used in low battery and it was removed when the phone is running the MMC can be corupted and the will be damaged. So the MMC should be formated. For that you have to formate via phone if not we have to done with PC. For that memory card reader must be used to formate the MMC. When we insert the memory card reader into the PC, and go to my computer and clik on the MMC icon which shown as removable drive in the in the PC and right click the option and it shows the options of FAT, FAT32, NTFS, in that 3 options select FAT. Other file systems will not valid to the MMC. After choosing the file system in the PC click on the format button.
Note :- Dont select the Quick Format option always select in Full Format option only.
And then the MMC was ready to use. If it shows some errors the it will be some Physicall damage.