I've been having similar problems with my SD card also. I have taking my sd card in and out of my camera in hopes that, that well take care the problem with no success. I than bought a brand new sd card and after puting it in my camera, turning the camera on, thesame screen keeps poping up, "card write protected" so, I came to a conclusion that the problem was not the sd card but the camera itself and here is what I did to fix the problem for good!!!
By the way, I have a canon digital camera rebel T1i.
In the camera slot where you would put the SD card in, if you look inside, by the way, you need good liting to do these, you'll see a gold tab that sticks out a little on the side wall, this is where the knotch of the sd card would go, the side that you lock or unlock; mine was jammed in and it was not spring out the way it should so that, when you insert the sd card, these gold prong, sorry I do not know what else to call it, but these prong would not spring back so that it would touch the white plastic tab that you slide to lock or unlock on the sd card. After looking inside the slot I took a very slender cleaning rod, it looks like the one a dentist would use for cleaning teeth with, and because it has a small bent at one end, I slided it in the slot being careful not to damage anything, and pull back on these prong and it sprong out, I than put the sd card back in the camera and to my surprise, it works just fine and the camera is in good working condition once more and I can take pictures again, I hope this helps....
George T.