I had this problem myself, solution was simply forcing the write protect switch to stay in place (I think I used tape).
The switch actually doesn't change the behavior of the card itself. The SD card slot has a little finger that "feels" for the write protect switch, and if it's in place it will disable writing. Problem is that when the switch begins to wear out, the finger that checks which way it's switched can actually push it towards the "lock" position. Tape or crazy glue will solve that.