A Digital Photographer's Worst Nightmare

In the case of a CardFlash formatted for an A-series Canon camera, the photos themselves are not erased from the card, even when you choose to format it! Their entries are cleared in the card's file allocation table (FAT), which means that new photos can be stored over the existing ones (or the space previously allocated to an old photo). If an old picture is partially overwritten by a new one, it's gone; if not, it's there and waiting to be recovered. This may vary a bit, depending on where the pictures you deleted are (in the beginning, middle or end of the sequence); those in the beginning and middle create holes that will be filled by comparatively smaller pictures, while larger ones will be located at the end of the sequence.

If you take a number of new pictures, though less than what you had before formatting the card, you can still recover the old ones! Er, good and not good.

Good: You can still recover some of the old pictures. If you accidentally formatted the card, or even only erased the card, all pictures are still recoverable.

Not good: Pictures you thought had been erased (erm, pictures you don't want anyone to see) are still recoverable, located after the new ones you took since the last erase or reformat.

If you really want to clear all pictures, format the card then take as many pictures of the same (safe) subject as possible as to fill the card. This will ensure that the whole card is filled to capacity with the same (safe, bland) picture. There may be a program out there that fills a card with random noise, but I am not aware of any. Alternatively, format it as a Mac volume, erase it, then reformat it as a camera volume (in the camera).

I tried MediaRECOVER and it does a wonderful job of recovering pictures from presumably 'erased' and 'formatted' cards many times over (surprsingly including very old pictures at the 'fringe' of the card that never managed to be overwritten by others). It also offers to format it as a Mac volume or camera card (I have not tried formatting it as a Mac volume). Well worth the money (I guess it was around the $29.95 mark).