Currently Being ModeratedJan 28, 2013 11:59 PM (in response to seanfromwilliamsburg)
It sounds like you have the external 42 inch display set in Extended Display mode. Pressing Command-F1 will change it back to mirrored display, so that it will display a copy of your desktop on your MacBook screen. The reason there is black bars on the side of the screen is because when you are running in MIRRORED mode, the external screen is displaying exactly the resolution of your MacBook. The 2010 MacBook Pro has a maximum native resolution of 1280 X 800. It WILL run other resolutions, but they will not look nearly as good as running natively. Your 42 inch TV is PROBABLY 1920 X 1080, if it is typical of most LCDs this size. You should be able to go into the settings for video, and change the output for the external display to THAT resolution, but your LCD on your MacBook will probably be black, since it is NOT capable of displaying that resolution. Once you unplug the DP adapter from your MacBook, the screen should come back on at its native resolution.