There are a few threads about this, both on this forum and the old/archived forum. Basically, Platinum actually uses 2 SAV slots, one is primary and the other is backup. It rotates between these slots to always have a current backup. The problem is that there is no utility that can restore both SAV slots to a retail cart. I've heard that if you save on the game, go into the computer and move a pokemon between boxes, then save again, both SAV slots should be about the same.
After that, it all gets fuzzy to me. Here's the best thread on the topic for further information.