Z2 G4 tower, i7 8700, one m.2 drive slot doesn't work

[Growly]

I know it's an old machine, but I have no reason to upgrade, unless this is faulty.

I have a Z2 G4 workstation. There are two m.2 drive slots, on the motherboard viewer diagram they are 15 and 19.

My m.2 drive in slot 15 works. I wanted to add a second m.2 drive but 19 doesn't seem to be working.  I moved my working drive from slot 15 or 19, and it's not recognised. The BIOS says there's no drive. 

Did the Z2 G4 only have one working m.2 slot? Is my motherboard 'faulty'? or is there some arcane BIOS setting I need to change to make it work?

Or am I going to have to buy a new, larger m.2 drive and clone the old drive to it, pretending I don't have a useless second m.2 connector?

[erico]

In the BIOS, go to Advanced, port options. There should be two M.2 storage options.  Click on the one that isn't already selected

[Growly]

Thanks for that, it was informative (I thought I'd checked all the obvious places)  however my BIOS doesn't look like that.

I will upload what my BIOS looks like shortly and perhaps we can figure out what's going on.

[erico]

Which version of the Z2G4 BIOS dop you have installed?

[Growly]

Thanks for your help Erico.
The BIOS saysQ50 Ver. 01.08.04 02/02/2023, under that it says 12.0.2.1088
A couple of screenshots of the BIOS.

I have an answer of sorts, although it begs more questions!
I think that the problem is the M.2 drive currently in the system.

I bought another m.2 drive to test. The existing drive works in one slot, not the other. When I add the new drive to the second slot, the old drive is no longer recognised. The new drive works in both slots.

So, it would appear the existing drive is the problem. Why it works in one slot but not another, and why the addition of another m.2 drive disables it, I do not know. 

[erico]

Kudos to your troubleshooting progress.

It does sound as if there is an issue with the first SSD.

I think it is a boot flag issue. Windows will not let two disks have a boot flag.

So with the first one installed in the original slot and the new one in the second slot, can you take a look at both using Diskpart to see if it is a boot flag issue? Have you given the second disk a disk letter? 

I highly recommend that you use the select disk command  first to ensure that the disk you want to remove the boot flag is the selected disk. 

You can remove the boot flag from a disk device by use of the Clean command in DiskPart.

Use the list volume command to see which disk is the boot volume.

Here is an image with examples of how I just  select volume 1