I recently had a compact flash card report it needed formatting in the camera, this was after filling it up with images.
I couldn't access the images at all, not even when I returned home to view them on the PC.
What I believe I had done to cause this was down to my technique at the PC with the last set of photos I had taken.
My method in RSE was:
I select all images on the media card, then move them to a file on the PC.
This empties the memory card of all of the images, leaving the usual canon eos folder.
However I forgot to format the card before re-using it.
I beleive the data left in the CF folder was telling the camera that there were still images there.
After filling the card to the point where the camera thought it was full, I could no longer use the card or view the images.
The camera returned the message media error, formatting required.
Recovery.
I downloaded a free program called
PC Inspector
This program supports most of the major camera RAW formats aswell as jpg, tiff, etc......
I set it running, backing up anything it found to my HDD.
After almost an hour it had recovered 140 images from the card.
One thing I did notice, the image number sequence was out.
Which is why I came to the theory above.
Conclusion.
Always format your card before using again in the camera.
Always transfer your images to your PC/Mac ASAP, then back them up.
Download PC Inspector just in case this happens to you.