Hi Carrol and welcome to the Q&A Board!
It seems pretty obvious to me that there is something wrong with the 512 XD card since it doesn't work but the other cards do. You may simply need to reformat the card in-camera to get it working again. You will, however, lose your images though there is always the possiblity of recovering them with software.
Since you can transfer the images to a computer, do so, then format your card in the camera. Make sure you format it and not simply use the erase function. Someone at this board recommended Art Plus Memory Card Safe Eraser. It's a free utility and many have praised its effectiveness. We have information about it here:
http://www.digicamhelp.com/free-digital-camera-software/index.htmIf this doesn't work you should contact the card manufacturer. There is a long, but very helpful thread in this section on XD card problems and solutions, which you may wish to check out.
Let us know how it goes.