Thats great that you got things to work! that inspires me to continue work on my 2009 Mac Pro 4,1 upgraded to 5,1. Maybe you or someone else here, have some tips from their experience. My system was working just fine, and booting up in High Sierra. Then one day the power light was on, but no power to the USB ports...no mouse or keyboard. So I shut down everything. When I started to power it up again, the power light comes on, but the system won't chime up any longer, and no mouse or keyboard. The light stays on.The fans run, the CD drive does it's thing, the HD's spin up, the graphic card fan goes on, the PCIe cards have power, the Processor tray blinks once, as it should. I swapped out the Power supply, no change. I swapped out the processor tray, no change. A new back plane board is what I originally replaced to get this machine to work. Checked the ram, pulled out everything not necessary, to get it chime-up. The power light comes on, fans come on, tray blinks. push the indicator button on the back plane board, everything lights up green & amber., No mouse & keyboard power, no chime up. Replaced the short ribbon cable that connects the Back plane board to the front USB/Fireware ports, no change. I suspect the back plane board, but I can't think of anything that could of caused it to go bad, since I replaced it.