You cannot use a normal MMC (Multimedia Card) or SD card.
It has to be a Siemens MMC (Micro Memory Card). // edit: This applies to S7-300.
edit: Not sure if you could possibly use a regular MMC or SD card in an S7-1200.
In an S7-1200 you have to use an SMC (Simatic Memory Card) that is actually compatible with SD cards as far as I know.