Just when the wether looks great both cameras die!!! Wondered if anyone has had this problem. My two month old 5d comes up with an error "cf card not formatted". I format the card in the camera. Still the error comes up. The same has now happened to my 400D. Ive tried a new card, still the same error but the guy in my local shop put the card in a new camera and it worked fine. The only common thing is the cards are swapped between cameras and i wonder if one of the cards has damaged both cameras. I also wonder if one of my cards is a fake. Damm.
I've never heard of a card causing electronic damage to a camera. It could cause physical damage I guess. Have you checked that all the pins in the card slots in the camera look OK with no bent or missing pins?
It does seem very odd that two different cards would both generate the same error on both cameras, but that the same card(s?) would be fine in a third camera.
If the card slots look OK and the problem persists there's not much you can do other than send the camera(s) to Canon for a checkup. The 5D should be under warranty, so I'd send that one first and see if they can find any problem with it.