Ugh, I felt his pain with a lost save not with FF III/VI but with the original NES Final Fantasy. I slogged through it and got to a particular point kinda far and saved it. Next time I played, all the data was gone. I tested the cart and it did save and it kept the save, so it might have been something funky when I turned it off or something. This was close to maybe 25 years ago or so when I got the game for a good price new (I think $20?). I didn't touch the game since.