I know this is not specifically a photography issue but since many of us with prosumer/professional-level DLSRs use Compact Flash cards, it is bound to happen to others as well.
I have a Sandisk 16GB Ultra card that occasionally had the camera reporting inability to access the card. I had to reinsert the card to continue using it. I was able to read the card and transfer the photo files to my computer drive. Today, the same thing happened again, just that nearing the capacity of the card, it failed one last time and the camera was subsequently unable to read it anymore. Despite reinsertion.
My other CF cards never once exhibited this problem, so it's pretty much isolated to this card.
The card is similarly unrecognisable on my computer CF card reader. I think the filesystem is toast, but I suspect the photo files are still intact. I would like to find ways where I can inspect the raw binary data in the card and hopefully recover the photos?