Failed to install bootloader

Hi, Sorry in advance for the wall of text; I’m trying to offer as much information upfront as possible. I’ve been running NixOS alongside Windows 10 on my PC since August 2025 with things running relatively smoothly, given that this is my first time running a Linux distro as my main OS. They’re installed on a system that I built in 2008; the motherboard is a Gigabyte GA-X58-UD5, with no UEFI support. Each OS is installed on its own separate disk, where there’s only one partition. NixOS is ins...