My experience with Mac Mini 2018, Sonnet 650w Box, RX580 and Catalina.
My ** experience ** is as follows, I am not a graphics guy. I was simply trying to get a machine that my son could do school work on (the Mac) and play Fortnite on at 100FPS (the Windows).
automate_eGPU fails * for me * on Catalina Beta 9. Even with a monitor plugged into the Mac mini. I had good luck with reFind. But I no longer use that.
All of my error 12 problems went away when I stopped plugging a 4K monitor - via HDMI - into eGPU and/or iGPU. I used an LG 2K wide-screen monitor instead.
Right now I can boot Windows (windows 10, 18326.295) and Mac (Catalina Beta 9). With a Sonnet Breakaway eGPU box connected to the left most (looking from front) TB3 port on a 2018 Mac Mini. The eGPU enclosure contains an AMD Sapphire RX580 (I had to return an XFX RX580, because it didn't work). Also plugged into the Mac mini is a CalDigit TB3 HUB ( I think this is creating a phantom screen, under windows), a 24tb raid array and a daisy chained 512GB ( Samsung X5 drive). This X5 drive contains the programs and other data for Windows. Plugged into the RX580 is a LG Ultra Wide 2K Monitor connected via DisplayPort and an OLD 20" Apple Monitor connected via a DVI to HDMI adapter and into an HDMI port on the Rx580. No monitor is connected to the Mac Mini directly.
When I restart the machine, I hold down the option and get the Apple Logo. Shortly after that I get the Apple 'Startup Disk' menu. Picking Windows I am booted into Windows (auto logon is turned on). Windows works well. Without issue. Fortnite on Epic Settings gets 80FPS. > 100FPS when Shadows are turned off. Don't ask me why we need 100FPS for Fortnite. I don't know. Everything gets warm, but there isn't excessive noise.
Now booting back into Mac.
When I restart the machine and hold the Option key down, I get the Apple Logo, followed by the Apple 'Select Startup Disk' menu. Selecting Mac I see various lights kick on. The RX580 spins its fans at full speed for a few seconds and I can hear the raid array spin up. Sometimes the result is as follows: the 20 DVI monitor will show a Catalina screen, and the main 2K monitor ( connected via Display Port) will show up white, or black. Usually white. I wait a couple of minutes and then unplug the the Thunderbolt Cable connecting the eGPU Box to the Mac ( I unplug it at the Mac end), and plug it back in, 30 seconds later the display shows up as you would expect and Mac OS Catalina can be used as normal. Catalina Beta 9 was still very buggy, but the monitors show up, the displays are eGPU'ed. All is well.
This process repeats for EVERY reboot. And works the same way each time.
I have not tried Beta 10 on this Mac mini.
I think there have always been issues with Catalina and Display Port connected monitor via an eGPU.
More testing is needed. But that is where things are right now.
Yes, I have the same issue with my RX570. Blank white screen after boot + my eGPU's fan spins at full speed and the GPU produces pure heat. An eGPU disconnect/reconnect solves the problem.
I think this is due to some kind of "handover" problem during boot. Cataline contains a firmware update for the Mac Mini 2018 which allows eGPU EFI video pre-boot and during boot, but something goes wrong when graphics accelleration kicks in once the boot process is completed. It would be interesting to reinstall Mojave and find out how nicely the upgraded firmware plays with that OS.
@steve4332 It may be GPU-dependence. I have tried my 2018 Mac mini with a monitor connected to the eGPU at boot but have not encountered this issue. The two cards I've used the most are RX Vega 56 Nano and Radeon VII. I will try RX 580 to see whether this is a bug with Polaris drivers.