Connecting external display turns MacBook screen black For years, connecting to external monitors on my MBP 9,2 (10.10.4) worked just fine (except the occasional crash). Until yesterday, where I had three crashes in quick succession. Now when the display (miniDP-to-VGA) is connected, the computer won't boot. I unlock my boot volume, but the black bar only fills to about one third before stopping. When I disconnect the adapter, it boots but once I connect the display, the laptop screen goes black and audio also dies after a few moments. The computer is still running and can only be shut down by long-pressing the power button.

I am baffled, but also unsure on how to diagnose the problem. Console shows no unusual messages.


I booted into my second OS (elementaryOS) and it reacts to connecting the adapter, but nothing shows on the other screen except occasionally a static image.
This may point to a hardware issue. Not sure. When mirroring the display, something does show, although I cannot select an appropriate resolution, which may be the tv's problem.
I reset SMC and NVRAM, to no avail
I also fixed permissions, although I don't know how that could fix anything, and it did not
I logged out and the same thing happens when connecting the display
I logged in as another user. When plugging in the display, the computer freezes, but the screen does not go black
In Safe Mode, the screen does not turn black and the computer does not halt, but nothing shows on the display, but I can access external display settings like resolution and such.
Could be a hardware related issue (either the MBP or external display) Similar issue happened to me and it turned out to be an issue with the LVDS cable. Also used to work at the Genius Bar, and would see this a lot with certain models of MBP. 

Try running a hardware test https://support.apple.com/en-us/HT201257 , or if you can take it in to the Genius Bar. They have diagnostics which will help isolate it to either hardware or software. 

Have you tried a different external display / display cable?