Formatting a drive or card reinitializes your disk (tables, bootsector, etc) by writing new values to your disk. It's "writing" to the drive, just as it writes to your drive when it saves pictures. So in essence, there is no difference as far as the mechanics goes, between formatting and just using the disk for storage. Format away! As far as not deleting files from your disk using your pc... if you ALWAYS reformat your drive when you put it in your camera, then it's ok to do.