The MMC/SD card doesn't care what file system is on it - thus the Sandisk doc does not mention it. As mentioned by Craig - see Microsoft for the FAT file spec. A shortcut that may work for you is to format the card on a PC and to write a file of 0's for the full size of the device. Assuming you only want to write one file you can read the directory for the first sector of the file and write sequentially from that point. How you remember where you last wrote is another issue - you can search for a sector of 0's on start up. Anyhow, this means you can read the file on a PC without having to worry about a file system on your embedded system. Extracting the data from this one large file can be done by a custom program on the PC. For embedded system with very small ram sizes (< 512bytes) you can also just waste space by writing small amounts of data per sector and just padding the rest of the 52 bytes. Again, you need a custom program on the PC to extract the data. These tricks may be of use to you.