Have a Skylake system running 10.13.6 (Build 17G6030) using a nVidia GTX 750 Ti graphics card. Loaned it to a friend and he used "sleep" mode once. After that, sometimes booting gave no video, sometimes it worked, but now I have the unit back and have been trying to find the problem.
Clover screen comes up (4920) at boot and both SATA SSDs are available. Selecting either one results in no display, but video signal is there... just black. Tried on-board HDMI and also on-board DVI in addition to the PCIe selection/port; same result. At one point the video came up normally, but when restarted, back to the black screen. SMBIOS is iMac 14,2. When up briefly, ran MultiBeast and re-installed correct options, but restarting resulted in the black screen again. Somehow lost the tonymacx86 Clover screen and now see the "embedded" display when Clover comes up.
So it's not a graphics card problem; seems more like a config.plist problem, but cannot identify since cannot get a display even though the OS is running.
I get about 1 second of a text display right after selecting the desired SSD, but don't know how to pause the screen long enough to get a picture.
Any suggestions? I've been working for about 10 hours and can't find the magic.
Thanks for reading!