Flash / non-volitile memory cards actually "wear out" and are not intended to last forever. The process of "reading" from a card does absolutely no damage to the card (you can read a card an infinite number of times), but the process of "writing" to a card is technically a distructive process. Each time you "write" data, it very slightly damages the card. This has to do with how non-volitle flash memory works.
Better cards will handle thousands of writes (often hundreds of thousands) before it's a problem, but all cards WILL eventaully wear out if used enough (that's a certainty based on how the technology works). You should think of these things like the tires on your car. After each drive you wouldn't necessarily notice that the tires are wearing out, but you know that if you drive the car enough, you'll eventually need new tires -- and it's not a defect of either the car or the tires, it's just the way things work.
I tend to stick with the better card technologies... since a bad card can ruin your whole shoot, it's not an area where I'm willing to save $5 by buying the cheaper cards.
Lexar and SanDisk brands seem to have a pretty solid reputation.
Keep a spare card with you so that, should you get this error, you can immediately pull the card out (the ability to recover the card depends on your NOT reformatting or over-writing the card with new data) and slip in the spare so you can keep shooting. I always have a few spares...
Tim Campbell
5D III, 5D IV, 60Da
Key word "after".
