Some time ago, a customer named Guy wrote in asking for help with a 24” white iMac he bought from a friend. The machine worked fine, save for its shattered screen, and he wanted to know whether he could plug in an external display and have it display the image that would have been on the internal display.
The quick answer is no. By default, plugging in an external monitor will result in an expanded desktop. You’d need to go to the Displays preference pane to modify the arrangement settings, and specify mirroring to accomplish this. I responded to Guy that the best way out is to replace the screen, at significant cost.
Some months later Guy sent me an email as a follow up. He had taken apart the iMac and unplugged the display cable from the display. When he reassembled it, he plugged in the external display, and lo and behold, it was as if mirroring was enabled.
So, if you find yourself with a Mac with a broken internal display, you can eliminate the issue of enabling mirroring by simply unplugged the display cable from the main logic board.
Thanks for the message Guy. I hope this helps someone out there!