I had this problem after replacing the screen on my Macbook Pro A1261. When I 'zapped' the PRAM Battery it solved this – Turn off Laptop, hold down - alt, cmd, P and R buttons and press power button – hold down until mac turns on, turns off and then restarts again. **I found other key combinations to use on older macs to Zap PRAM are Shift,Ctrl,Alt and power buttons, Seán