Well, it did it with a brand new card. So, put brain in gear.... low-in case anyone is interested and it happens when I try to use an image directly from the card.
We cooked some really beautiful octopus a couple of evenings ago. While in the marinade, we thought to get a shot of the dish-who knows when one might open a restaurant and need menu shots?-and rather than save it to the hard drive first, just was going to email the shot using Apple Mail. It hung up the computer. Did a Force Quit, and voila! card is no longer readable.
So, for whatever reason, now, I save to hard drive first! then use the image.