iMac mid 2011 reboots when connecting an external display
It is safest to treat DisplayPort as not hot-pluggable.
Only connect or disconnect when the computer is off. Power on the display before the computer [it may go straight back to sleep, this is not a problem, it will re-awaken during boot.]
It theoretically should be capable, but relies on the computer, display &amp; cable to all be compliant [&amp; not buggy].
Google &quot;display port hot plug&quot; to see some of the issues it has.