iMac (Mid 2011) and later: Target Display Mode does not work with Mini DisplayPort cable

Symptoms

When you attempt to connect an iMac (21.5-inch or 27-inch, Mid 2011) and later to another computer with a Mini DisplayPort to Mini DisplayPort cable, the screen of the target iMac will not become an external display when you press Command-F2 to activate Target Display Mode.

Resolution

iMac (21.5-inch, Mid 2011) and iMac (27-inch, Mid 2011) and later computers support Target Display Mode via Thunderbolt to Thunderbolt cable (2 m) when the source is another Thunderbolt-equipped computer.

iMac (27-inch, Mid 2011) computers do not support Target Display Mode via Mini DisplayPort cables. 

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