Mac Pro 4.1 (flashed) High Sierra Firmware Update

high sierramac pro

I just created a USB Installer for High Sierra and tried to install my Mac Pro 4.1 (flashed to 5.1).

But the installer wants to do a Firmware Update too and says I have to shutdown and press the power button until the EFI beeps.
But this Shutdown button in the installer is greyed out so I can't proceed.

Any help?

Best Answer

Okay, I got it.

First, I installed Sierra an a USB drive and did all updates.

Then I inserted the High Sierra USB drive and started the installer. And voila, I was able to press the shutdown button and did the EFI update!

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