I am at my wit's end, so I decided to post here to save myself from further headaches, in case I'm missing something obvious.
I decided to purchase a 512 GB M2 NVMe SSD (the Samsung P981), but since my kinda-outdated Motherboard (Gigabyte GA-970A-D3) doesn't have such a port, I decided to connect it via a PCI-e 2x4 to M2 NVMe adapter (MAKKI-M2-PCIE-VE1). And proceeded to install Manjaro on it. The installation went well, no errors. However, my motherboard can't seem to see the adapter as a bootable drive, and neither can GRUB. Both of my installed operating systems (linux and windows) have no trouble seeing and writing to the drive, as does the Manjaro Live USB image (obviously)
-Attempt 1: Installed Manjaro, with the bootloader (grub) all on the SSD. Not bootable, so this attempt ended there.
-Attempt 2: Installed Windows 10 on the SSD, with the bootloader of my other windows. Gave me an error about system32/boot.efi being corrupt or missing. Tried to copy the one from my working windows installation (a bad idea, but still worth a shot) to no avail.
-Attempt 3: The previous attempt gave me an idea, what if I were to install grub to a bootable drive? I installed it in a 512MB partition on my other drive, and it was visible as a boot option, however, GRUB threw an error about not being able to find the drive it's supposed to boot from.
I tried both BIOS and UEFI mode with GPT partitioning table, nothing changed. Also tried fiddling with some settings in the bios like the secure boot option to no avail.
My guess is that the adapter I used is not a bootable one (doesn't have the firmware to tell the bios what it is), but the OS's have some kind of driver for it, so they can access the SSD.
Someone suggested I try Clover bootloader, which I tried installing onto a USB, but I was unable to boot from it (and I had a headache at that point anyway), and I'm not about to replace my system's bootloaders with something I have no guarantee it works. I could give it another shot if someone thinks it would work. I tried using SuperGRUB CD instead, but it couldn't see the SSD either.
Does anyone have any suggestions before I lose it and return both the SSD and the adapter to the seller for a refund?