Case You have a 500d (or any other camera, using a SD-slot), can happen, that the camera does not feel the inserted card, or gets the false information, that the card is write-protected.
The cause is in most of the cases, that the two small switches, one for carde presence, and the other for write protection are damaged. To replace a card slot ONLY for this reason is dangerous for the board and a lot of work. Normally i rather make a bridge on the switch – a small wire and 2 solderings.
Her you see how it looks for the 500d. Just check the right soldering pafs on the PCB.