My early 2009 Mac Pro that was running 10.12 for months without issues, bricked after trying to install 10.13 (I know, I was stupid to update). I downloaded High Sierra and the first thing it did in the installation is updated the flash. The update went ok and it rebooted fine. It then started a 40 minute installation and when I returned, the front panel LED was on, the fans were turning, but I have no power on the USB ports (keyboard / mouse) and nothing on the display. Cycling the power on gives me some noise from the optical drives but nothing else. Does anyone have any suggestions or am I'm SOL .