Make sure you do it on your computer and (in windows 7 anyway) uncheck the "quick format" option. From my experiments so far, I've seen that even exFAT works, so if you're going to go through the trouble of doing a slow format, you might as well get the higher file sizes allowed by exFAT. Now, I've only used exFAT after I had already installed ML on my camera, so I dunno if you can run "update firmware" on your camera with an exFAT formatted card, so be wary of that. I'll be doing some further experiments this week to see what I can get away with.