Anyone could politely share with me any reminiscences on why it may be impossible to USE an iMac in target display mode? It’s happening to me with three different iMacs I’ve got here (I could on another two, but these aren’t the fitting 27inch ones I need to test an Mac Pro 2013 with graphic misbehaviour): This is all I’ve tried (as suggested by Apple and forum users): Pressing Command + F2 several times All have OS X 10.6 or later (which is a requirement) I used the appropriate known good cable needed (thunderbolt for mid 2011 or later iMacs, minidisplayport for earlier 27inch iMacs) I reset the PRAM and SMC Logging out then in again Sleeping then resuming again Checking "Use all F1, F2, etc. keys as standard functions keys” is disabled in system preferences-keyboard. Starting the iMac AFTER the source mac with the cable connected to both. Using a good known aluminium Apple keyboard on the target iMac. Starting them from an external HDD with Yosemite.