Originally Posted by Slowpoke Rodriguez 
I have been using my K10D for about a year and have always used a Lexar 2GB SD card in it. Over the past month or so, it has started to tell me that it doesn't have a card in it (when it does) or that the card needs to be formatted. I had been able to work around this by turning the camera off, taking the card out, putting it back in, and turning the card back on. This little work around has now stopped working. It just doesn't recognize my Lexar SD card at all anymore.
I put in a new Kingston card and it sees it, so far, but this is only one day of use. When I put the Lexar card in my card reader, my computer can still pull the pictures off the card, and I can still read and write to it.
Any thoughts?
I've seen this before. It tends to happen with cards that aren't formatted regularly.
Try this:
Since your computer can still read the card, use your computer to do a full format.
Then try putting the card back into the camera and formatting it in camera.
It is very important that cards be formatted regularly (I format every time I put a card in the camera), and it is imperative that the card be formatted in camera.
If that doesn't do it, repeat the process.
Cards corrupt, and they fragment really easily. They need some attention from time to time.