I've been running Windows for my cMP for many years while it was supported form the Boot Camp Assistant. Then in 2017 things changed to prevent the cMP 4,1 from running Boot Camp. So I found a work around from YouTube that involved flashing the PRAM. During so with its required software provided by the YouTuber helped to get my 4,1 to run Boot Camp again. Since my system was taken from a 4,1 to 5,1 status. So I was able to update Boot Camp Assistant to change my partition I had years prior. Then as of 2018 the cMP 5,1 was also placed on the chopping block as like the prior cMP's before it. Which didn't concern me since I had already created a Boot Camp partition years ago. However after the update to Windows 1903 something gone wrong with the boot up after the upgrade. I ran into blue screen issues and the startup repair that Windows 10 has wasn't able to solve the issue. Which lead to me scraping the partition and using a SSD.
These are capable machines so I don't know why Apple thought it was necessary to prevent them from updating. But since seeing the new cMP introduced this year it's clear what Apple's intent was targeted towards. That said MacOS Majove is the last supported OS for the 5,1 cMP. The next MacOS coming later this year has new hardware requirements. Something not stricken as form the Developers Beta yet. Apple will purposely required the software to check for components to make sure that neither the old Macs nor any Hackintosh builds will be able to install the final release. I believe the T2 chip might me that wrench!
FYI, If you have a PC ONLY Graphics Card (non Apple Graphics card) installed in your cMac Pro you will NOT be able to see the Boot Menu by holding down the Option key at startup. However you can select from with in MacOS from the Start Disk menu. Getting back to MacOS is where things get complicated with a Non Apple Graphics card. Flashing the PRAM becomes the only option. And Apple doesn't plan to fix Boot Camp for the cMP. So people like myself decided to go all in with WindowsOS. Truth be told Windows 10 version 1903 included all of the drivers for my 2009 Mac Pro that has been software flashed to 5,1 status. I didn't have to install a thing. For my installation to a larger SSD I removed all the hard drives and SSDs I had inside my cMP. All except the one I planned to have Windows 10 installed. Prior to the removal I reformatted the hard drive to Fat32 (which is very important). After the install all worked well. Only thing I done was update AMD software that controlled my MSI ATI Radeon RX580. I haven't had any issues and it's been nearly a week.
