Do you want to reformat it? If so when on the desktop click Go>Utilities>Disk Utility Open it up then select the card (presumably in the card slot on your Mac), then "erase". I leave mine set to FAT32.
That file is a great idea, I will definitely add that. I found a Phantom 1 with a go pro crashed in my yard last year, which is what piqued my interest in quads. I turned it into the police department as found property because I had no way to identify an owner. I finally got my first quad yesterday, P2V, and had the maiden voyage...and crash.. Tonight. Back to the card, when I drag the files to the trash it tells me the files can't be deleted.