1 day later, sill trying to get HS on my Mac Mini, it's now in a state where I can't boot into anything except the High Sierra Install, I have no wired Apple Keyboard, no startup button presses are registered on boot up time from my Bluetooth Apple Keyboard, so it's more or less in a boot loop, it says it fails, I have to hold the D key to try to solve the disk problem yet nothing works.
I will try again with a bootable MacOs High Sierra USB stick.
Tried to get it to install from a MBP in Target Disk Mode, got a message it's not allowed, hell, I did that many times before.
Can't even start in Target disk mode anymore, otherwise I would just copy my disk to the internal SSD on my Mac Mini.
Eventually I will get there but this is not the way I remembered it to work.
First time ever it went wrong, and I have been on MacOs/OS X since the very start.