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When working in the embedded world there are some chances that you will come across an application requiring greater memory storage space than is available in most microcontrollers and even external EEPROMs. That's why SD or MMC cards are great. They are cheap and easy to work with. For most applications all you need is an SPI port, and even if one is not available you can easily emulate one on most microcontrollers. With the SPI port you can communicate and store data to the SD cards, but it gets a bit trickier. Most applications will require you to store or retrieve data from a personal computer. So you need a way to connect your application to the computer and more importantly a method to organize the data in a way that will be recognized by the computer’s operating system. For this you need to have some sort of file management system. Chances are you will use the FAT 16 or FAT 32 system. The File Allocation Table (FAT) system is somewhat simple and supported by almost all operating systems. It is better to understand how it works before adding it to your embedded device.” source...