I was recently given a used early 2011 macbook pro. (my sister didn't want it anymore because it wasn't working very well I guess) I checked the memory under "about this mac" and noticed that only one of the sticks of ram was recognized. It had 2gb of ram in one slot and the other slot read empty. I assumed that it had gone bad and decided to replace the ram and upgrade it at the same time. I installed two 8gb sticks which should have left me with 16gb total. but when I checked the the slots again, the computer only recognized one slot and indicated that the other one was empty again. Does anyone know what's wrong? Is it possible that maybe the slot itself is broken? I'm running OS X 10.9.5 Software issue? Motherboard? I'd appreciate any advice, thanks!