I recently bought a Benq 4K 32" monitor and am running into an odd and annoying issue... the 4K monitor is not detected by the OS at start-up, resulting in a black screen w/ "signal not detected". The GPU that I'm using is a flashed Nvidia GT 630 2GB (bought at macvidcards), which supports 4K @60hz. I'm running High Sierra on a Mac Pro 3,1... so yes, I'm technically into "unsupported" territory. And due to the EFI, my GPU won't show the grey screen/Apple logo at startup when using 4K resolutions, which doesn't bother me. In my testing, if I start the Mac up using my older 2560x1400 monitor, get to the log-in screen, THEN move the Mini DisplayPort cable over to the 4K monitor, the signal shows up fine, and I have 4K @60hz. But rebooting with the 4K monitor still hooked up results in the "Signal not detected" message. Starting up with both monitors attached vid DP (the new 4K & my original 2560x1400) works; although the 4K stayed black until after boot, it was readily recognized by the system nonetheless, and acted as a secondary screen. So... the GPU can handle both screens at the same time, but for some reason the Mac refuses to recognize the 4K on its own...? I even tried resetting PRAM, but with the 4K connected, the Mac refused to complete the process (no chime... at all... until I released the key combo). Yet the PRAM reset worked fine with the older monitor attached. It just makes NO sense to me, and while I'd love to keep the 4K, screw it if OSX can't make it run correctly. Any ideas before I start the return process?