1) Do you have any PCIe cards installed?My SSD is in SATA port 1. All other ports are empty at present.
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I have tried using the Install High Sierra downloaded via the App store. I have also tried using a USB thumb drive with the Install Hugh Sierra on it that was set up to be bootable. The MP 5,1 will not recognize the thumb drive as bootable however. Possibly because it will not boot from High Sierra without the firmware update?
I have pondered a clean install of Sierra on the SSD. I have tried everything "logical" that I can think of. I have only resisted the flashing of the GTX 680 because I am not really a PC person. While I did build a Windows gaming machine last year, I am not conversant with Command line prompts or the Windows OS beyond very limited basics. In fact, I had built that machine to be a hackintosh but never felt comfortable getting the macOS on to it. Remembering the correct connections to the power supply and attaching the GTX 680 to it for flashing purposes is just uncomfortable. I am not sure I even have the right cables to do it. I really am not inept. This problem just confounds me.
2) Do you have a empty HDD?
You could install El Capitan from the Mac App Store on a empty disk, boot from it, download 10.13.6 and then try to do the firmware upgrade.