So if I keep the .dat file of a game on my comp and delete the game off of my card, can I put the game back on my card later and relink it to the old .dat (save) file? How do I do this if we can?
yes, it will work. just make sure the file name of the rom is the same as the save file, differing only in the file extension. and some backup devices also need the save file to be in the same directory as the rom. not sure about m3 minisd, but i do know that backing up just the save file and when restoring, renaming the rom and save file to match works with all devices.