Thank you for the reply.
With Kernel Issues I wouldn’t know any remedy if there was one.
Boot Mode is excluded, since the machine has only UEFI mode.
Firmware/BIOS Settings is rather truncated on UM790 Pro and there is not much to play with. I don’t see what would be wrong with the way it’s set, considering, that UM780XTX has almost the same settings and I’m running StartOS on it after upgrading the BIOS to the latest version.
Corrupt EFI Variables: is there any way to detect/identify it? - I don’t know.
As far as formatting, I did what you’ve suggested: as I said in my initial post, after I erased and formatted w/GParted first, I installed Linux Mint on one drive and wiped another one (did w/each drive), using the same command. I then tried it with clean disk and the formatted one with ext4 - nothing worked, StartOS just wouldn’t install. I also inserted the disk with SrartOS installed on UM780XTX but UM790 Pro didn’t see it.
Minisforum replied and suggested to ask StartOS support - so much for that.
I will try it again with the wipefs command and if the same persists, will wait for the BIOS update - might do the trick just like for UM780XTX.
Thanks again.