Answer to "iMac restarts when external monitor unplugged"I have exactly the same issue with my 27 inch iMac 2011. I connect a Apple LED Cinema Display and the moment I remove the usb and mini DV port the iMac reboots then goes into a rebooting cycle. As soon as you reconnect the led monitor the iMac reboots correctly and runs perfectly. It’s really strange. Did you ever get to the bottom of your issue?