Formatting the cards in the camera (and ONLY in the camera) is a good habit. If you format the cards in Mac or PC, you might experience some problems. Formatting the card in a computer can be tricky, since different operating systems perform formatting slightly differently.
In addition Mac writes some hidden files to the card after formatting. It also writes these files even when you just insert the card into the Mac computer.
When you format the card in the camera, the formatting is always the same and in addition the camera creates some directories for files and an index file. To be precise, the camera not only formats the card, it also prepares it for shooting.
Therefore I always follow some strict but easy rules: copy the files to computer, make a separate backup, and format the card in the camera. It always worked till now in it never failed.
Happy shooting, Niko ----------------------- Never underestimate!