Why Will 2012 Mac Pro will work with 2 graphics cards, but not one? I have a 2012 Mac Pro which must be updated to a Metal compatible card. When I swap the cards, the screen shows nothing and I do not believe it boots. When I put in the original card and the new card however, it works fine and video works from both cards. The problem is, the Mac will not allow a Mojave update with both cards in, and when I remove the original card, I am back to the same issue regardless of which x16 slot I use. What could be the cause of this?
It appears the problem was that FileVault was turned on. Even without Mojave, some graphics cards without special firmware (although approved by Apple), are unable to render when in FileVault. This is why Apple says to turn it off before moving to Mojave, but they do not mention that it will not work with their new graphics card recommendation. Therefore the order is very important


Turn off FileVault
Install new card
Update to Mojave