(Like I said, I put a Mac 2.66 GHz Quad-Core Intel Xeon Processor card, 24 gB of DDR3 ram, and a newer WiFi card, to make it as compatible with orginal specs as possible)
I had restored the 480GB SSD from APFS back to HFS+ yesterday, and did a restore via time machine to Sierra 10.12.6
Last evening I started a fresh Install High Sierra (From a fresh dload copy in my Applications folder) - scheduled to take 2 hrs. so I walked away, had dinner, etc. Came back later & the machine was off - restarted and it looks like it had formatted automatically to APFS and gotten hung some time after that - Bad drive symbol. Did a reformat back to HFS+ and restored from Time Machine. Running Sierra 10.12.6 again.
This is pretty much what happens every time I try to upgrade to High Sierra -
it happened during the Beta period, happens now with the final release.
I've tried with a Seagate SSHD [1000 GB hybrid drive], and it happens with the OWC 480gB SSD.
I've no idea how to proceed now - and technically, this machine as mac Pro 5.1, should be a simple upgrade.
And - Mojave is on the way !!