I just started the process this morning with my 4,1=>5,1 Mac Pro. With original HD4870 I updated firmware to 089 and then fully installed 10.13.6 (decided to not try and install Mojave on Sierra), all of which was no problem.
I then installed my Mac EFI flashed HD7950 and it re-booted into HS and ran also without any issues at all. Looking good I thought. So I started the Mojave installer which wanted to update the firmware and after re-starting it with the button pressed in etc, I left it alone to update the firmware.
When I returned, the screen was black and the ACD monitor just occasionally flashing, but no other apparent activity and no way I could find to get the screen to work. I waited as long as could and then killed it with the button and re-booted into the previous installation backup. All seemed ok with an Apple logo and the progress bar on screen, but then it just went black again. I left it for a long time until disk thrashing sounds had subsided and tried to log in blind. This seemed to have no effect and I was wondering what to do now. But then, some combination of keyboard and mouse actions (and maybe some arcane incantations - ok, swearing
brought the screen to life. I was indeed logged in to the previous install on the backup disk. But I have no idea what it was I did that kicked the screen into life. However bootrom was now showing as 138, so the firmware update had indeed worked ok.
With everything now working, I re-started and logged into the desired volume containing the recent update to HS. Got to the end of the progress bar and although it did go black briefly (maybe it always does) I then had full display of my HS desktop and everything seemed to be ok. So...
I then proceeded with the Mojave install. No complaints as the installation got started. The screen has gone off a few times (although no re-boot) and the grey screen is now back with the progress bar showing some time still remaining. Only time will tell if once finished I will have a working display.
So that's it so far, but I was intrigued by the previous post about flashed HD79xx cards giving trouble. Could someone provide any more info about this? What is the supposed trouble and is there a fix? Am I perhaps suffering this exact problem?
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Well I'll be ...
As I was about to post this the screen when black, then returned grey with progress bar a couple of times, then black again and came back with proper login screen. I logged in and I am now looking at my new Mojave desktop and my fusion boot drive is now APFS. So it does seem to have worked (so far
. So why the black screen hiccup in the middle?