Even on an Apple backups are not optional so you should be able to slip in a single new drive to test if the prior working OS works fine. I don't see a ready document about High Sierra and supported machines but for now, why not run the OS that works?
A couple weeks ago my mid-2010 Mac Pro started getting extremely slow at times. Restarting would temporarily fix it, until one day it wouldn't get past the apple logo progress bar screen. So I took it in and Apple ran a diagnostic and told me that it's unlikely to be the HDD but "something else" that wouldn't be worth replacing, and recommended a new computer.
However, my old HDD won't migrate to the new computer. Good thing I have a bootable backup. It takes longer than it should to mount, it's extremely slow in accessing files and makes a repetitive sound in even intervals like it's trying to load. So I started thinking that maybe the problem was just with the HDD.
I'd prefer to stay with my old Mac Pro and swap out the old hard-drives with new ones if possible. So I put a new HDD in it and tried installing macOS high Sierra from a flash drive. However, it only gets to the following screen at which point there's a problem..."To install macOS high Sierra, a firmware update is required. To complete the update, click "shut down..." The shut down button is grayed out and will not let me click to continue the process.
Is this clearly a bad sign or is there still hope to install an OS and stay with the Mac Pro? Is it possible that my old nearly-full HDD triggered a more serious issue, or vice versa? I'd really appreciate some guidance.