Black screen and stuck reboot with eGPU solved in 10.15.4
I updated to 10.15.4 and can not boot without getting the black screen. I still have to boot up CPU and then after 20 seconds power up the eGPU. After upgrading to 10.15.4 I am also getting some screen flickering as well.
2018 Mac Mini (6-core i7)
With the caveat that I (once again) forgot to unplug my eGPU while updating - not sure if that is part of my issue - I can cold boot with 10.15.4 without having both an HDMI cable from my mini to my display and my eGPU connected. No Apple logo, but it also doesn't hang without ever getting to a login screen. This probably still doesn't really help if I want to boot to Windows, as the lack of a logo screen probably means I also wouldn't get a boot device screen if I tried it.
Restarting the mini still hangs without HDMI, regardless - no login screen ever appears. Wish I would have remembered to unplug the eGPU during the update, just to know whether that is still causing mischief.
Yes you need to update with the eGPU disconnected or you are back where you started
I can confirm I updated with the thunderbolt connection and eGPU off and now it works totally fine for start and reboot
I tried with the combo update (they're available from https://support.apple.com/downloads/macos) and the eGPU powered off, but still end up with the black screen about half the time when cold booting without HDMI plugged in, and still can't get it to restart without hanging at the black screen. Since the Apple logo never appears via the eGPU, even when it finishes booting correctly, this probably isn't a big help, since it probably means that the option key boot menu won't appear either, so I'd still need to have HDMI plugged in for that. Maybe in 10.16 this will all get resolved?