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I will explain it later on because I have to go to work now. It's kinda involved and requires a bit of a long-winded explanation I think.

It works!!

Driver Installed and detecting the GPU

I got it booting with the 1660 Super Installed! It's not the most elegant solution... I will install the drivers and see if I can get Plex transcoding working. If it works out, I will post here my solution.

I tried inserting a Windows 10 install DVD to see if I can get it to boot into that. Holding option presents two options... Windows + EFI Boot. Selecting the Windows option locks the system, Selecting the EFI Boot option brings me to a black screen that says "Press any key to boot from CD or DVD....." And then just hangs there.

I've done a little bit of further testing. 1) I reset the PRAM/NVRAM by holding down Option+Command+P+R at the startup chime, and kept waiting until I heard the third chime. 2) I held down the option key at the chime to get the apple boot menu. With the Apple GT 120 the boot menu shows up with one option: the DVD with plopkexec. It appears as a Windows disk and pressing enter will boot it. With the 1660 Super, I get the same boot screen, but pressing Enter will not boot the disk - It just stays stuck on the boot screen 3) I removed the 1660 and put it into my Windows PC - a fairly recent Lenovo Legion T5 tower and the GPU seems to be perfectly fine. So I think I can rule out a bad GPU.

I just found this interesting thread on MacRumors about why there is boot screen support on the Mac with the 1660 Super. https://forums.macrumors.com/threads/rtx-series-cards-have-native-bootscreen-support.2148023/ Alas, no proper drivers for the Mac side though, but at least that's one mystery solved!

Hey, thanks for the reply. Yes, I thought it was odd too, which is why I decided to comment here. What do you mean by "look at how you're booting plopkexec"? At first I was using an external Apple USB superdrive with plopkexec burned onto a DVD. When that didn't work with the 1660 Super, I put the internal back in and tried the dvd there. I'm really not sure where else to go with this. Any pointers would be very helpful!

Hi, I was able to get this to work on 4,1 flashed to 5,1 Mac Pro with Plopkexec 1.6. Everything is working swimmingly, but once I put in a GTX 1660 Super GPU into the machine, it will no longer boot. It just shows a light grey screen and a folder with a question mark on it. I have no idea where to go from here. If I remove the GPU, it boots as normal. To be honest, I was surprised it showed any kind of picture at all. Any help would be greatly appreciated! Edit: I tried plopkexec 1.6 64-bit, 1.6 32-bit and 1.5 64-bit with frame buffer drivers and none of them will boot.