No, don't choose the Install from the USB flash drive. That just needs to be selected once.
Thanks for hanging in there with me pastrychef ... The next time you are in Germany, the first six pack is on me =)
I finally understood what I did "right" during the first installation process, but decided to abandon a second installation on a separate partition and have gone back to tinkering with the first install, which resides on a partition that I didn't erase. Why?
It turns out that the first install was not corrupted during installation: the "MacOS Install Data" wasn't related to an incomplete or botched installation of 10.12.6, but to my having used Migration Assistant to migrate docs, apps etc. from another drive, including unwittingly parts of 10.13.6 used in an attempt to create install media from an interrupted App Store installation of 10.13.6 per Tonymac and Apple Support instructions on how to do that. Since that didn't go so well, I decided to stick with 10.12.6.
So, the first troubleshooting I did was to go back (no bios upgrade, bios changes, etc) and just replace the EFI folder with a pristine copy of yours ("High Sierra"), then tried to boot up from the HD disk it's attached to, and lo and behold, it worked!!!!!!! Just like that! USB stick not necessary any longer!! =) That's a solid EFi you've made for us. I must have read something in a different post and then made an alteration that messed stuff up.
I am sitting in front of a screen with 10.2.6 running off my Seagate drive. Can't believe my eyes ... I'm doing a reboot now just to make sure I'm not dreaming .... yeah , it's real, it booted again. Picts to follow ... Now, the good and the bad news ...
Here's what's working so far (the Good):
--all ram recognized
--Nvidia 1070 mini (haven't installed the second one yet, but I don't expect problems)
--sleep, reawakening etc, no problem
--Belkin pci wifi card works too (but may eventually want to go your way on the wifi connection, especially because an installed 2nd 1070 will make it impossible to also populate the smaller pci with the Belkin)
So far, here's what's not working (the Bad & Ugly):
--no audio: itunes not working, nor quicktime: no music progress bar moving, no sound
If I remember correctly, the audio is supposed to work "out of the box", isn't it? Any suggestions? Maybe something I can add, or take out?