MP 1,1-5,1 Downgrade Problems (From Mojave to High Sierra)

[DJ Dilbert]

I have a 2010 5,1 Mac Pro. I recently upgraded my hardware to 2, 6-core x5690s, 64GB ram and a GTX 1060 6G. My next move was to upgrade the OS to Mojave, but I didn't know that Nvidia's web drivers were not supported in Mojave. So per Apple Support's instructions, I swapped out the Nvidia card for my original ATI 5870 and attempted a time machine restore to my SSD to downgrade back to High Sierra. Note that all my hardware was running perfectly when before I upgraded the OS.

TL;DR, I screwed something up and I'm not sure what. I now get a flashing folder when powering up (suspecting the OS got corrupted). I also cannot boot into recovery mode either, nor can i do internet recovery or select a disk to boot from (for my USB boot drive for High Sierra).

I'm fresh out of ideas. Please let me know if i need to provide any additional details. I can still create files on my Macbook Pro if need be.

Thank you in advance!

 

[tsialex]

Install the AppleOEM GPU, use the boot picker to boot from a createinstallmedia USB High Sierra installer, fully erase your main drive, reinstall. After everything is updated, install the current NVIDIA web drivers and finally change the GPU.

 

[DJ Dilbert]

Install the AppleOEM GPU, use the boot picker to boot from a createinstallmedia USB High Sierra installer, fully erase your main drive, reinstall. After everything is updated, install the current NVIDIA web drivers and finally change the GPU.

I tried that. I let the installer run but it just hangs on the initial progress bar. I thought maybe it was just slow as it's over a USB 2.0 connection, but I let it run overnight and I woke up to it still just hanging at the full progress bar. I'm not sure what is wrong.

 

[tsialex]

I tried that. I let the installer run but it just hangs on the initial progress bar. I thought maybe it was just slow as it's over a USB 2.0 connection, but I let it run overnight and I woke up to it still just hanging at the full progress bar. I'm not sure what is wrong.

Enable verbose boot and see where is stopping for any clues?