In case people were still doubting it, the one save slot thing was completely confirmed in the new interview from Famitsu. Basically what's said is that because of the higher degree of freedom you get in the game, there's a ton of data that has to be saved, so there's only enough space on the cartridge to store one hard save, and one quick save. There's also a kind of confusing bit in the same part that talks about making a backup of your save in whichever space isn't being used. I don't really understand that part, since it makes it sound like there's no real difference between a quick save and a hard save. Perhaps someone who's read the interview can give some insight on that.
