So I had used a 24" old cinema display with adapter to mini display port for years with my old 2010 iMac 27"...Worked great. Finally upgraded to a late 2015 5k 27" (refurb) a few weeks ago. Works great. When I first hooked the Cinema display up to the 5k it worked fine, then it wouldn't work, display just stayed asleep. Moving mini-display adapter from 1 TB port to the other results in no change. TB port works fine (both of them) because I can use a TB to Firewire 800 adapter to get my firewire drive working. So I decide it is monitor....It is ancient. So I buy a 4k 27" Dell display. Plug it in, worked great. One reboot later, instant power save mode, nothing SMC reset, nothing PRAM reset, nothing. What the heck is going on??? Overheat? Bad hardware? Isn't making a lot of sense to me.