The rear brick is made from a natural mineral called Vermiculite which is an awesome insulator and will eventually break when exposed to the kinds of temperatures that you find in stoves. It expands as it gets hot and so will inevitably crack and wear away during normal usage of your wood burning stove. A crack will reduce the efficiency of the stove a tiny bit, but is safe to use and not at all unusual. Keep your eye on it when you empty or clean out the stove (when it is cool) and when it is wearing thin give me a call to arrange replacing it.