First issue would be why only one card? If you follow Jen's very logical advice, you will use multiple cards - lose one or it takes a hit you don't lose the entire shoot/trip.

If you have been using the same card over and over, have you ever formatted it in the camera (other than when it was brand new)? Have you tried to read the card with a stand alone card reader (best $15 you can spend at Sam's Club)? Can you view the existing images on the card?

Cards are dirt cheap. Buy a new one and try it in the camera to see if it works. That would be a good indicator as to whether the card or camera was the culprit. Even though you will be using a different card type in the D50, it is worth the $15 a card should cost you to determine where the error is. Of course, you could also borrow a card if someone else uses the same style, but be sure any images on it are copied to a computer and backed up before you try it in your camera.

If the card is the culprit you may be able to salvage the images on it if you have recovery software. If you are willing to lose the existing images, have you tried to use the camera tp format the card?

There are other potential reasons why you might be having an error. I know of at least one person who had a bent pin in the camera causing issues.