Hi, I have a Mac mini 2018 , with a Sonnet Breakout Box 550. In the Box is an AMD RX 580. In Mojave, I would boot the Mac partition, and only connect the RX 580 to the monitor. after login, I had full acceleration and was truly plug and play. I never plugged in the HDMI cable to the Intel GPU. In Catalina, the Mac would not boot to complete the install without the built in Intel GPU connected. It just kept shutting down. When the Mac finally booted up, It showed 2 displays .. both being powered with the Intel GPU. It took an hour of messing with it, finally I selected Mirroring in the intel screen, and disconnected it.. took a second.. RX580 is finally enabled. I'm in Catalina now, doing system tasks. I will do a full system back up, then see if Rx 580 survives a reboot. P.S. I'm really mad because drive icons are all wrong! I had it so pretty ! lol Sorry about my meandering composition, I'm writing this before my second cup of coffee. Thanks all!