Here is a function reference for Microchip’s memory disk drive file system library.
This library interfaces a PIC microcontroller with FAT16 or FAT32 file systems. It allows you to interface with storage mediums like USB flash drives or microSD cards in the same way a PC would. Save data on a storage device, and later dump it on a computer without extra hardware.
Of course, there are plenty of open source FAT implementations too.