NVMe M.2 problems, mobo related

[Snapshot]

I'm failing to get two different M.2 NVMe SSDs seen on two different Asus motherboards and I'm starting to get quite grumpy about it. But this is my first time using M.2 drives so there may be something I don't know about them.

The mobos are an Asus H370-I with i5-8600T and an Asus B310-I with Pentium Gold G5500T.
The M.2 SSDs are Plextor 128GB SATA, Samsung PM981 256GB NVMe & WD Black 256GB NVMe.

Both mobos fire up and can see any external SATA drives attached. They both have the latest BIOS.
Both mobos see the SATA Plextor drive so seem to be working but neither can see the NVMe drives in the BIOS.
I've proved both the NVMe drives work by putting them in an external USB enclosure and, before that, the PM981 had been RMA'd back to Scan and returned with no fault found.

I don't believe I have two faulty mobos so, apart from the fact that Asus mobos hate me, what's going on? I've been building PCs for almost thirty years and have rarely been this stumped.

PS although this is my first post here, some of you may know me from OcUK & TP forums.

[kalniel]

I found this post talking about BIOSes not seeing NVMe SSDs until they have a boot sector:
http://www.tomshardware.co.uk/answer...-bootable.html

If you have more than one M2 slot, it might be worth trying a different one.

[Snapshot]

Thanks for that and I'll try it a little later - once Mrs S is out of the house and I can use the table as my workbench.

[Snapshot]

And thanks again as it worked perfectly. I now just need to decide what's going in which box and bolt everything together.
But the NVMe SSD is still not seen by the BIOS even though the mobo is booting from it! This must lock out any OS that doesn't have built-in NVMe drivers which seems a bit naughty to me.