I have a 13" MBP and it won't detect my external display. It works fine on my sister's whitebook using, but won't detect at all on the MBP. I've went to "detect displays" under system preferences, played around with the cable, and made sure the monitor is on. It work's flawlessly on my sisters. What's wrong? Any suggestion on what I should do? I'm losing my mind right now. The screen on the MBP goes blue for a sec when the external display is connected, but then just goes back to it's original desktop.