I have a Mac Pro early 2009.
Updating from High Sierra to Mojave was a big pain and it started trying to find a compatible graphics card.
From info online i wasn't sure but at the end i decided to take the risk and order a Sapphire Pulse 11267-19-20G Graphics Card RADEON RX 560 1500MHz PCI Express
Choose the best. Choose the model Saphir Pulse Radeon RX 560 2G, 2048 MB GDDR5 Beyond performance it is the design and quality of manufacture that are at the appointment. The Radeon RX 560 Pulse version 2 GB by Sapphire: New in the AMD Radeon RX 500 range, the RX 560 Pulse relies on the AMD Polar...
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I replaced the graphics card with the RX560 and the Mojave installer stopped giving the Metal compatibility notice.
It asked me to update the Mac Pro Firmware. You need to press the Shut down button from the installer, it will ask to install a helper and it will shut down.
Then keep the power button pressed for 15-20 seconds until the power light flashes and then a long beep will sound.
It opened the cd rom after a minute. This signifies that the Mac Pro firmware has been updated.
For me, it had to be done twice. I don't know why.
Anyhow, then all i got was a black screen.
It would boot up and give a black screen. Nothing to do. I tried to reset the Nvram by powering off down the mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R
I did this many times but it did not help.
I put back my old graphics card in and tried to boot. No signal on the screens.
I put in again the RX560 and this time it booted.
It continued with the Mojave installer for about 30 minutes and all went smoothly from there.