I have an iMac (Retina 5K, 27-inch, Late 2014) running Mac OS 10.12.6 with a second display attached - Dell Ultra HD 4k Monitor P2715Q 27-Inch Screen LED-Lit Monitor.
Everything is good but every once in a while the iMac does not wake properly and I have to do a hard reboot. The Dell screen goes to sleep and the iMac screen stays black.
The display is connected with a Cable Matters Gold Plated Mini DisplayPort (Thunderbolt Port Compatible) to DisplayPort Cable, Black, 6 Feet.
I'm pretty sure that the problem is related to some sort of handshaking (detect displays) between the Dell and the iMac.
I have tried just about everything included a reinstall of the OS, but I still find myself having to reboot the Mac every day or two.
Besides this one headache, the monitor is great - but if I have to, I may just purchase another secondary display. Anybody else encounter this? Are there any suggestions for fixes or for a display that is more compatible?