No Video Output on Second Screen During Startup? I have a 2018 Mac Mini with two screens attached; one is a 4k screen connected via HDMI, the other is an old 20" Cinema Display attached through a USB-C to DVI adapter.

Now, when started up this works great; the 4k screen looks amazing, and the old cinema display is a handy second monitor for throwing references, YouTube videos etc. onto to avoid cluttering my main workspace.

The problem is, during startup the Mac Mini only seems to want to output video to the cinema display; the 4k display receives no video at all. The only way to get output to the 4k display is to ensure the cinema display is physically disconnected during startup; even if I disconnect its power from the wall socket, as long as the USB-C to DVI adapter is connected to the Mac Mini, macOS still considers that monitor to be connected.

It also doesn't seem to matter how I setup the twin monitors; for example if I set the cinema display to mirror the 4k display (not that I want to do this) then I still only get output to the one screen during startup.

So my question is, is there some way I can set HDMI/my 4k display as the default for video output during startup? Some NVRAM setting or such?
The 'one screen at boot' is pretty standard behaviour, afaik. I see it on all the Mac Pros here. 

Which screen that ought to be is dependant on which is set as Primary in System Prefs > Displays > Arrangement - it's the one with the menu bar, which can be dragged to another display to set Primary.



That it still displays the same behaviour with one screen switched off is because it still recognises it as being attached. When you detach it completely, then basically you get an "Oops, Primary is missing, let's find one we can use temporarily."