macOS Catalina breaks Sonnet eGPUs with Radeon 580x cards

I should have never upgraded to macOS Catalina on my 2018 Mac mini. Mojave was, for all intents and purposes, stable and smooth. Catalina, on the other hand, has broken my eGPU. (For what it’s worth, I held off upgrading from Mojave to Catalina until 10.15.1.)

The problem that I’m running into is this. With the Mac mini plugged into the Sonnet Breakaway Box eGPU (with a Radeon 580x card installed), I am unable to get the Mac mini to boot to the macOS login screen. The machine seemingly hangs with a green screen and some weird magenta video artifacts. The green screen is coming from the display directly connected to the HDMI port on the back of the Mac mini. The other two displays, a Dell 2515H monitor and a Wacom Cintiq 27QHD, display a series of magenta lines and a black display, respectively. (See images below.)

The workaround that I’ve found is to unplug the eGPU – where upon the login screen appears. Once logged in, plug in the eGPU. Suffice to say, this is unacceptable. I’ve filed bug reports with Apple but there is no ETA on when the bug will be fixed. From what I’ve found on the Internet, the problem has something to do with the Polaris-based card inside the eGPU.

Here’s hoping Apple fixes the problem soon.

-Krishna

Update: As of this writing (2-27-2020), Catalina 10.15.4 beta1, beta2 and beta3 all fix this bug.