Some users are reporting Mac mini 2018 boot issues with an eGPU attached. The problem that happens to them is seeing a black screen from the beginning. Having ruled out a connectivity problem with cables, screens or different Mac outputs, everything seems to indicate that the incident arises from the security of our Mac, specifically from FileVault.
The definitive test is carried out when you originally have two Mac mini from 2018, one with encryption enabled and one that does not. Important, that this differentiation of the encryption occurs from the beginning.
The problem arises when we boot our Mac mini from 2018, we see the black screen due to FileVault security, which is really asking us for the encryption password. Without entering this password, the system does not finish booting. This happens since the external graphics processor starts at the same time as our Mac. As the screen is black, we must enter the password, blindly!
As possible solutions, it is valued to connect the screen directly to the Mac mini, without having it connected via eGPU and reserving a port for other peripherals. Still, Apple security, as soon as it detects a connected eGPU, disable video outputs again, returning to the black screen.
The funny thing is that this problem only affects the Mac mini from 2018 and not to other Apple products, apparently with the same structure and security, this is the T2 chip. In the forums they comment as probable, the integration of a monitor in other equipment such as the MacBook Pro or MacBook 2018. The results seem to primarily affect the Blackmagic eGPU, but other users report problems with the Sonnet eGFX Breakaway Box eGPU as well. In addition, it affects all types of connections, both to screens with HDMI and USB-C.
Only two days after the release of macOS 10.14.2, it is to be expected that Apple is working on an improvement of this behavior in the 10.14.3 or a specific update for Mac mini 2018 and correct this discomfort.