If you return the memory card to the helmetcam, it may choose to reformat it before use; back to some FAT-16 or FAT-32 format many of these third-party cards and cameras use. Unless you were going to use it for Mac OS X and format it with HFS+ or in a partition map for Mac data, etc.
If not, the easiest way to reformat the card may have been in the camera itself. I've formatted several kinds of camera cards, drives, USB flash drive, and others. The means varies with the intended purpose. Sometimes the default is the best; since it is platform neutral if the device is seen by both players.
Good luck & happy computing! 🙂