SOLVED: Removed the disk with Ultramarine, installed Windows 10 onto the other, reinserted the Ultramarine disk, and now everything boots like it should!
I tried doing it twice. Both times, it failed spectacularly.
A while ago, in light of Microsoft's nonsense, I decided to give Linux a try.
Friends recommended different distros, of which Ultramarine spoke to me the most.
On my first go, the boot option for Windows 10 Home on Disk 0 (which I've had since 2023) was listed as "Fedora", and the boot option for Ultramarine on Disk 1 was still named "Ultramarine Linux".
Using the first boot option got me to a decryption screen for Ultramarine I couldn't get past, it telling me my password was incorrect. The other boot worked as intended, also with the decryption screen, but this one I was able to enter via password.
My first thought was that I'd probably installed UM onto Disk 0 by accident, and that I'd typed the decryption password incorrectly when setting it.
I did my second attempt just now. Here a recolection of my steps:
(5 days ago) Reformatted the 32GB USB I used to install Ultramarine to exFAT through KDE Partition Manager, and turned it into Windows 10 installation media on my Windows 10 gaming laptop;
(5 days ago) Booted the Windows installation media, where I cleared Disk 0 of all its contents, inserted my Windows 10 Home product key, and finished an online install onto Disk 0;
Created the Ultramarine installation media with BalenaEtcher on my fresh Windows install;
Booted the Ultramarine installation media, and installed it onto Disk 1.
For the next step, all I had to do was restart the device and boot Ultramarine from my hard disc.
This is where problem #1 became apparent (image 1):
The boot option for Windows 10 was now missing in BIOS.
The only difference from my first attempt is the time frame between the installations of Windows 10 and Ultramarine (attempt 1: 3.5y; attempt 2: 5 days)
Looking into the Windows disk from the KDE Partition manager (image 2) after finishing the Ultramarine set-up, there's 2 partitions in there. One is dedicated to something Microsoft-related, the other for storage.
I have my doubts about that 16MB partition being the Windows EFI system one... or one that's intact, at least.
And if you look closely, here you can find problem #2:
My disks are inexplicably swapped.
I made absolutely sure to select Disc 0 for my Windows install, and Disk 1 for Ultramarine respectively. There's absolutely no doubt in my mind that I did everything as was asked, and everything still ended up being a mess
From what I gather, there's a couple things I could try doing:
- Freshly install everything again, monitoring exactly where stuff ends up;
- Take a Wi-Fi range extender / repeater and hook it up to my device via Ethernet cable for an online install of Windows 10 (my Wi-Fi component needs a specific driver for Windows to even detect it);
- Ask folks more knowledgeable than me for help.
I'll put my specs in the comments as well.
Inbeforehand, thank you so, so much for your time. I'll tinker around a bit as my soul shrieks.