I have encountered several problems with my CF cards before, but they've always been solved by formatting the card (either in the camera or the computer).
However, this time, I inserted my card into my D70s and it showed 'FOR'. I tried to format the card with the camera once, but the sign still remained. So I put it into my card reader. On the computer, it displayed 'the card has not been formatted', and subsequently prompted me to do so. The computer formatted the card, but when I put the card back into the camera, it still displayed 'FOR'.
I tried to format the card on the camera again, but this time the green light kept on for a very long time (around 3 minutes). However, it still showed 'FOR'. I've tried to take out the card and put it back in again, and sometimes it shows 'CHA', but usually 'FOR'.
What should I do? It's a Sandisk Ultra II 4GB and usually has no problems on my D70s.