Cover card open when it's shut

This problem is caused by the little piece of plastic that you flip down to hold the XD card in place. It has a nipple sticking out of it that is prone to breaking off. Unfortunately, this nipple (about 1/16" x 1/8" x 1/8") holds down a little button which tells the camera that the the plastic piece has been flipped down. Once the nipple breaks off, you need to find some other way to hold the button down.

I simply took a small plastic peg from an old boardgame -- a cribbage-board peg might work -- and whittled it down to fit the opening. Remove the XD card and you'll see the small 1/16" x 1/8" rectangular opening. There's a tiny, squarish black button inside. Use a toothpick or tiny screwdriver to carefully press the button in while the camera is on, and you'll see the error message disappear.

Once you get the idea, use a tiny Phillips screwdriver to remove the plastic piece that you flip down. There are two screws to remove, two little metal retaining brackets and a tiny metal rod. Be careful not to lose any of these pieces! Then gradually whittle your piece of plastic so it fits the rectangular opening snugly, and consistently turns off the error message. Then cut it to about 1/8" in length, insert it into the opening, and reassemble the rod, brackets and screws to hold it in place.

Now the little button will always be pushed in and the camera will be functional once again. Since the XD card has its own spring-loaded "locking mechanism" (you push to insert and push to remove), the piece of plastic you flip down isn't really needed in the first place. You can save it, anyway, just for the memories....

There is usually a small tongue on the card cover that engages either a switch or some other device to detect if the cover is open.
Check that it is not broken or that the sensor is not damaged etc.
Hope this helps...