Sorry for the lack of updates lately, I had very little time to continue working with the laptop.
Yesterday, I temporarily removed the 250 GB M.2 SSD with the broken Windows 10 in order to test whether I could boot from the Win10 USB now. After removing the SSD, I started the laptop once into the BIOS and then shut it down (I suppose, this was the suggested UEFI mode boot?). And indeed, I seem to be able to boot from the Win10 USB - I got to the language selection menu and then to the 'Install Windows 10' screen, so I think I could proceed with installing Windows 10 Pro. I stopped there for now.
Next, I tried to boot from the Windows 7 USB in order to set up the intended triple boot - unfortunately, with the same result as always (as posted in #15:
https://forums.tomshardware.com/threads/windows-10-boot-error-kmode-exception-not-handled.3768755/#post-22736340 ). I made a clean new bootable USB drive in order to rule out any hardware problems with the one I had used before, but the result remains the same.
My procedure for creating the Windows 7 bootable USB:
- Make ISO of a Dell Windows 7 Pro x64 SP1 DVD
- Create a bootable Windows 7 USB drive with Rufus
- Use the Gigabyte Windows Installation tool to add USB3, NVMe drivers & packages (like here):
As I had previously written in this thread, I could intiate the boot process from the source Windows 7 DVD in an external DVD drive (USB connection). At some point though (when the 'starting Windows' screen with the Win7 logo appears), the connection to the DVD seems to stop, and then the laptop freezes, with no visible progress (at least not within ~30 minutes). But this is still the furthest I got with trying to boot Windows 7; USB has never worked.
I even tried once to start the Windows 7 installation (from DVD) from the running Windows 10 system, and while it would copy the files and initiates the required system restart, the result would still be the same - the 'starting Windows' screen with the logo appears, and then it's frozen for all eternity. And I've been stuck with a broken "Windows Setup" in the boot selection menu in the first position since, so whenever I was not paying attention to actively select Windows 10 instead, I would enter the broken setup again. I couldn't remove the broken entry at all - it's still on the 250 GB SSD with the non-functional Win 10.
Strangely enough, the issue with booting from a DVD also occured when I first tried to install Windows 10 from a DVD: I could boot into the menu, select the target drive to install Windows 10, even create the target partition (this is where I noticed that Windows 10 always creates 2-3 additional, smaller partitions next to the target one), but when I hit install, an error prompt would tell that the connection to the source medium is not found.
Any ideas on what might still cause this issue? What should I try, test or change next in order to proceed? Did I do the suggested UEFI mode boot correctly, or did I overlook something?
And is there maybe a clean, trusted Windows 7 Pro x64 image from a safe source that I could try instead of mine? I know that Microsoft stopped supporting Windows 7 a long time ago, but I recon there should be a mirror somewhere...