Cant re-boot if external monitor plugged in

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I am having problems on boot/re-boot - mac book doesn't reboot if external screen is connected.
Its a 2009/Running Mountain Lion

I just get a grey screen. I have to unplug the external monitor (mini dvi) and plug back in.

Any ideas?
 

chscag

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If you're operating the MacBook on battery it may or may not reboot with an external monitor plugged in. I used my white MacBook in clamshell mode for almost three years and it too would not boot with the external monitor attached if I operated it only on battery. I understand the new model MacBook Pros will reboot on battery with an external monitor attached but I have no way to test that.
 

chscag

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Check the mini DVI adapter and cable going to the external monitor. If possible, swap them out. Also try resetting the System Management Controller on your MacBook. See the following Apple KB article to learn how: LINK
 
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MacBookPro 11,2 <--DAW!!!
I forgot which key combination actually does this but on Boot up, try cmd-opt-t-v or cmd-opt-a-v to use an external monitor.
 

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