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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?

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    Which cards do you have and what version of macOS do you currently have installed? – Bob Apr 1 at 21:18
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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

  1. Turn off FileVault
  2. Install new card
  3. Update to Mojave
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Sounds like the new graphics card does not have a Mac video firmware. That is essential in order for the graphics card to be supported during boot.

After boot, ordinary drivers for the graphics can be loaded and the card starts working letting you use both the new card and the old card.

You must replace the firmware or entire card with one that is supported during boot on a Mac. Whether you can update the firmware depends on the type of card.

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    If you don't have a Mac-flashed Metal-capable card you must disable FileVault & any pre-boot requirement such as a firmware password. – Tetsujin Apr 2 at 6:22

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