I run 10.13.6 on a 3.1 Mac Pro. Before I had a GTX 1060 in here, but by accident I installed the update to 17G3025 so that card stopped working. Due to the absence of new Nvidia drivers I decided to sell this card and purchase a RX 580. Also because I wanted to upgrade to 10.14 Mojave later on. The exact device I purchased: Sapphire Pulse Radeon RX 580 4GD5 (11265-09) But now, when I start up my computer, the screen stays black. (I knew no bootscreen would appear) And where usually the login screen appears, I get a white screen for a few seconds. After that it gets dark again for a few seconds, and white again and this goes on forever. Luckily I have a Bootcamp partition so I could check if it worked in Windows. That worked like a charm, so no issues at all. But I can't find a way on how to get it working in High Sierra. What I tried till now: - I had a 6-pins pcie connector plugged into the 8-pins on the videocard, so I added a 6-pins to 8-pins adapter so the other two pins had power too. Still, no difference. I know a proper 2x 6-pins to 1x 8-pins adapter should be better, but since it works in Windows (also when gaming) I don't think the power is an issue. - I checked the product ID (0x67DF1002) in windows and checked if it was listed in the kext (AMD9500Controller.kext), and it was. - Filevault is OFF. - The original BIOS is loaded, also checked this with the seller. - I checked all ports: DVI, 2x DP, 2x HDMI - they all result in the same slow blinking screen. (however, when using DVI the white screen is a bit green) - Enabled screen sharing, and tried to see if I would be able to connect to it via vnc. I got a login prompt once, but couldn't connect. - Added an old 8800 GT, together with the RX 580 and tried to boot. I got the bootscreen, and the progress bar but that never finishes and I got into a boot loop in this configuration. I'm not sure what to do next. Does anybody has an idea? Any help will be really appreciated.