Suggest a new class 4 Sandisk card. Android (which the Nook OS is) works better with this class 4 card than others. Check out Amazon -- they're prices are quite good.
I'll second what PJLLB said.
SD Cards are notorious for failing quite regularly. Sandisk is a good company - their SD Cards are more reliable than most.
Also 4X is the speed that works best with Android Tablets. Nook is based on Android so it too works best with 4X speed.