Athough most operating systems these days are FAT32, I agree with rinniethehun
in that it's always best to format a card in the camera to ensure 100% compatability with the camera
. (Cards used touse FAT-12 (up to 64MB) and FAT-16 (above 64MB), but as card capacity has grown, they've started using FAT-32.)
In fact, I never delete used images, I always re-format at the start of every new shoot; thismakes sure that the card is clean.
Always make sure that you've transferred your previous images before formatting.