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Here's a question for you from the Mac Pro subforum
Build no. 18A353d
That won't work with a RX580. You'll have to use a GPU with EFI (bootscreen-capable) for the FW-Update.
To clarify, Betas are displaying the build number in 'About this Mac' permanently. Final releases don't, you have to click on the version number.
First semifinal is coming closer
R9 280X works fine, see #28 here. DP2 and PB1 have the same build number.
In case you missed it
So, got DP2 up and running on a 4,1/5,1. Can report a flashed 280X works fine, isn't laggy either.
I didn't use a supported machine. I used...