I’m going to assume you have Windows 10 on a USB flash drive, either the one you imagined yourself, or one of Microsoft’s official USB drives. Depending on the age of your Windows 10 USB key (mine dates from 2015), the installer will try to create an MBR partition for you that will not start on Mac Pro 5.1. I assume that if you have a newer installation version, it will configure an EFI NTFS partition for you, so you will not need to do this next step. Install Paragon or Tuxera (none is free but you will find it useful later, I used Paragon.) Configure your NTFS drive partition from Disk Utility in macOS. It could be one of your SATA-2 connected drives or it could be a PCIe connected SSD, I used the latter. Be sure to choose Format: Microsoft NTFS and Schema: GUID Partition Map, not Master Boot Record or Apple Partition Map.
At this point, you’re ready to install Windows, but unfortunately, you’ll have to put your flash video card back on Mac (assuming you have one) because Windows 10, even modern versions of it, won’t show any type of video for example on an RX 580 (that’s what I have.) on Mac Pro 5.1. Turn off your Mac, reinstall your Mac flash video card, and boot from your USB drive. It should appear in the text menu to which I referred earlier. If you did not flash your bootloader at the first step, it’s ok because you probably have a Mac firmware video card installed here so you can keep the option and boot that way, it will be more difficult to return to Windows later. Install Windows as usual, it will copy the files, restart, and then you will have an option to run Express Setup. Choose Custom Configuration and answer No to all invasive privacy options: No, No, No, No => Next => No, No, No, No, No => Next. It will restart once more, ask for your username and password, and you’re done with the Windows 10 installer.
Windows 10 should now be installed and you should be started on Windows. You will not be able to start in macOS at this point and Windows will start automatically. Don’t panic, you can return to macOS in a moment. We need to install some drivers. If you’ve created a Boot Camp USB drive from the start from the High Sierra Boot Camp wizard and have since upgraded to Mojave or Catalina, don’t bother with that, as it’s too old for the panel Boot Camp Control. read your APFS disc. If you are running High Sierra on HFS + and do not plan to upgrade, you can skip this next step. Install 7zip from here: https://www.7-zip.org/ and Brigadier from here: https://github.com/timsutton/brigadier/releases. Once Brigadier is installed, you want to run a few commands to download the iMac Pro Boot Camp drivers. The instructions are here: https://crystalidea.com/blog/bootcamp-and-windows-10-1903-may-update. The basic instructions are as follows: load a command prompt run as an administrator, run `brigadier.exe –model iMacPro1,1` to download boot camp drivers for iMacPro1,1 which include how to read APFS discs, then run `msiexec / i BootCamp-041-55643 BootCamp Drivers Apple BootCamp.msi` to install it. The exact command of the second part has changed slightly, so you will have to modify it, but you will be able to indicate the version of the command to execute from the file downloaded during the first step. `brigadier.exe –model MacPro7,1` might also work at this point, but I haven’t tried it. This will install the Boot Camp drivers you need to be able to select your macOS drive in the Boot Camp Control Panel.
With the Boot Camp drivers installed, you can now run Windows Update to get the latest version of Windows or download it from the Microsoft support page if your Windows 10 installation is too old for Windows Update to work like mine. We’re almost done, but we still need to install drivers for your Polaris or Navi based video card so that Windows 10 will display the video on your upgraded PC card. DO NOT install the AMD Adrenalin 2020 Edition drivers from the AMD site as they will NOT give you video. Instead, download the drivers from https://www.bootcampdrivers.com/. You want those from Download => Windows 10 … => 2020 Drivers … => Adrenalin January 2020 Red Gaming Edition (Best FPS in Games) or whatever the latest version is there if you do it at to come up . Do not use drivers labeled Mac Legacy (prior to 2013) … even if the Mac Pro 5.1 is older than 2013, it may use a video card of the year 2013, which is why you don’t want not install that one. I suppose you can also run the Adrenalin January 2020 Red V2 Gaming edition (Best for emulation and non-gaming software) or the Adrenalin January 2020 Blue Enterprise Edition (Best for 16-inch MBP and Radeon 500), but I installed the Red V2 Gaming edition version (Best FPS in games).
Once these drivers are installed, you can shut down your Mac and reinstall your Polaris or Navi video card, boot into Windows, and the video should now appear. Open the Windows Control Panel, go to the Boot Camp Control Panel and select your macOS drive as the startup drive and reboot to return to macOS. You can restart in Windows from the text-based splash screen that you probably installed at the start.
I’m going to assume you have Windows 10 on a USB flash drive, either the one you imagined yourself, or one of Microsoft’s official USB drives. Depending on the age of your Windows 10 USB key (mine dates from 2015), the installer will try to create an MBR partition for you that will not start on Mac Pro 5.1. I assume that if you have a newer installation version, it will configure an EFI NTFS partition for you, so you will not need to do this next step. Install Paragon or Tuxera (none is free but you will find it useful later, I used Paragon.) Configure your NTFS drive partition from Disk Utility in macOS. It could be one of your SATA-2 connected drives or it could be a PCIe connected SSD, I used the latter. Be sure to choose Format: Microsoft NTFS and Schema: GUID Partition Map, not Master Boot Record or Apple Partition Map.
At this point, you’re ready to install Windows, but unfortunately, you’ll have to put your flash video card back on Mac (assuming you have one) because Windows 10, even modern versions of it, won’t show any type of video for example on an RX 580 (that’s what I have.) on Mac Pro 5.1. Turn off your Mac, reinstall your Mac flash video card, and boot from your USB drive. It should appear in the text menu to which I referred earlier. If you did not flash your bootloader at the first step, it’s ok because you probably have a Mac firmware video card installed here so you can keep the option and boot that way, it will be more difficult to return to Windows later. Install Windows as usual, it will copy the files, restart, and then you will have an option to run Express Setup. Choose Custom Configuration and answer No to all invasive privacy options: No, No, No, No => Next => No, No, No, No, No => Next. It will restart once more, ask for your username and password, and you’re done with the Windows 10 installer.
Windows 10 should now be installed and you should be started on Windows. You will not be able to start in macOS at this point and Windows will start automatically. Don’t panic, you can return to macOS in a moment. We need to install some drivers. If you’ve created a Boot Camp USB drive from the start from the High Sierra Boot Camp wizard and have since upgraded to Mojave or Catalina, don’t bother with that, as it’s too old for the panel Boot Camp Control. read your APFS disc. If you are running High Sierra on HFS + and do not plan to upgrade, you can skip this next step. Install 7zip from here: https://www.7-zip.org/ and Brigadier from here: https://github.com/timsutton/brigadier/releases. Once Brigadier is installed, you want to run a few commands to download the iMac Pro Boot Camp drivers. The instructions are here: https://crystalidea.com/blog/bootcamp-and-windows-10-1903-may-update. The basic instructions are as follows: load a command prompt run as an administrator, run `brigadier.exe –model iMacPro1,1` to download boot camp drivers for iMacPro1,1 which include how to read APFS discs, then run `msiexec / i BootCamp-041-55643 BootCamp Drivers Apple BootCamp.msi` to install it. The exact command of the second part has changed slightly, so you will have to modify it, but you will be able to indicate the version of the command to execute from the file downloaded during the first step. `brigadier.exe –model MacPro7,1` might also work at this point, but I haven’t tried it. This will install the Boot Camp drivers you need to be able to select your macOS drive in the Boot Camp Control Panel.
With the Boot Camp drivers installed, you can now run Windows Update to get the latest version of Windows or download it from the Microsoft support page if your Windows 10 installation is too old for Windows Update to work like mine. We’re almost done, but we still need to install drivers for your Polaris or Navi based video card so that Windows 10 will display the video on your upgraded PC card. DO NOT install the AMD Adrenalin 2020 Edition drivers from the AMD site as they will NOT give you video. Instead, download the drivers from https://www.bootcampdrivers.com/. You want those from Download => Windows 10 … => 2020 Drivers … => Adrenalin January 2020 Red Gaming Edition (Best FPS in Games) or whatever the latest version is there if you do it at to come up . Do not use drivers labeled Mac Legacy (prior to 2013) … even if the Mac Pro 5.1 is older than 2013, it may use a video card of the year 2013, which is why you don’t want not install that one. I suppose you can also run the Adrenalin January 2020 Red V2 Gaming edition (Best for emulation and non-gaming software) or the Adrenalin January 2020 Blue Enterprise Edition (Best for 16-inch MBP and Radeon 500), but I installed the Red V2 Gaming edition version (Best FPS in games).
Once these drivers are installed, you can shut down your Mac and reinstall your Polaris or Navi video card, boot into Windows, and the video should now appear. Open the Windows Control Panel, go to the Boot Camp Control Panel and select your macOS drive as the startup drive and reboot to return to macOS. You can restart in Windows from the text-based splash screen that you probably installed at the start.