Here it goes

The BIOS still looks exactly the same as in my pic. It shows only two disks.

In the meantime, someone pointed to me that it is the SSD, and not the M2 that is undetected by the BIOS (the M2 serial is displayed in the BIOS). Why did I believe it to be the M2? Well, for reasons that make this story even weirder! When I first entered Windows after I installed the M2 and the SSD only one aditional disk was showing. I tested it with Crystal Disk Mark in an effort to find out if it was the SSD or the M2 and I got a writing speed of around 500 MB/s. I tested the system disk and I got a speed of around 3000 MB/s, so I assumed the recognised disk was the SSD. After I had finished these test a third disk suddenly appeared, so I assumed it was the M2.
I have just tested the three disks and oddly both the M2 and the SSD have similar speeds, while the system disk that came with the Nitro is noticeably faster. Apparently, this is a slow M2.

Enclose please find the speed tests.

"What happens when you remove the Western Digital m.2 low speed SSD? Does 2.5" low speed SSD then show up in the BIOS?" - good question, but I'm not going to open the laptop a third time so I'll probably never know. I get nervous when I do it, fearing I do something wrong and ruin the equipment. Especially with the 2.5" SSD which uses a very flimsy connector that required a lot of fiddling before it finally clicked in place.

Thanks for the explanation about the sockets.

"The speedy BIOS POST routine must be correctly recognizing the missing 2.5" drive, but for some reason the drive hasn't printed its own model & serial numbers on the screen" - I didn't know that was even possible.

In the back of my mind I have this fear that I didn't connect the SSD properly (the connector is minuscule and extremely difficult to deal with), some pin isn't connected and that's what's causing the problem. But that doesn't make much sense, does it? Either it is 100% connected or it isn't connected at all, right?

This is interesting. On this YT video this guy installs an M2 and an SSD on a Nitro 5. He sees both of them in the BIOS, but the factory installed M2 doesn't show up?!?! Check out min 9:30 in the video. think there's a bug in this BIOS:

The guy on that 11 months old YT video has answered really fast.

My question: Hi! I've just installed two disks on a Nitro 5 and only one of them is showing in the BIOS, just like what happened to you at 9:30 . All disks show up in Windows though (which shouldn't be possible...). I'm curious, did you ever manage to make all the disks show up in the BIOS or does this BIOS just have a bug? Thanks.

His answer: Hello thanks for watching. I did get them to show up once I reseated the ribbon cable. Maybe just double check that. Perhaps it was connected a little weak and didn't show in bios then was detected after bios. hope that helps!

So apparently the problem might be down to just a cable that isn't properly connected..But I don't think I want to deal with that incredibly small and fiddly miniature connection any time soon. Maybe when/if I buy a bigger (2TB) SATA 2.5 inch drive. I've installed a 500GB drive because that's what I had lying around in some drawer.

I think this BIOS will accept a 2TB drive, right?

I've tried both LInux Mint and Ubuntu running from a pen, I just didn't install either of them. But I know they work. I've also tried one of them (I think it was Ubuntu) running from RAM (using the TORAM instruction) but I had some problems, maybe I did something wrong.

Anyway, please check what I've posted just before your post, apaprently this whole problem may be no more than a flimsy cable that isn't properly connected.

Five years ago I had smoke billowing out of my son's computer (literally) after installing a new graphics card… Miraculously only an Adata SSD was burned. But the smell lingered on that PC for ages.