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First, Ignore the Dell-Cares message.  As far as I can tell, it's a write-many-read-never account.  No mattter how many times you reply, it will ignore you and keep sending you messages.  As to your problem, you can get a cheap ($20?) PCIE nvme adapter.  Or you can buy a used Dell Quad Ultra-speed (about $100?). If your machine allows partioning you can use 4 nvme cards with the quad.  Note that the PCIE card splits out the channel into 4 subchannels.  The following might be useful.:https://www.urtech.ca/2019/11/sovled-video-dell-ultra-speed-drive-quad-nvme-m2-disk-pcie-adapter-with-windows-storage-spaces/

I think that there is a solution that uses the slots in the front of the machine.  IIRC that was way more expensive and not as fast (????)  

More info for you here:

https://www.dell.com/community/Precision-Fixed-Workstations/Best-way-to-install-an-nvme-in-an-T7910/td-p/7772362

Good luck

Hello WizardOfBoz2

Thanks for your idea.

I've followed the advice :

nvme disk doesn't appear as bootable disk .

After has booted on ubs stick, nvme is seen as non bootable device by windows.

I've booted to nvme via both a cheap, $17 PCI-E card and also a Dell Quad adapter.  Maybe something in the BIOS settings? Can anyone with better knowledge help here? 

You have a tower, OP has a rack.  Did not install boot drive for R7910 before, so I can't say for certain.  But it doesn't hurt to give it a try.  Usually, it takes around 15 minutes to install Windows.  Why not just proceeding with installation to see if it can boot.

For hardware and settings, just a passive PCIe to NVMe adapter will do.  No special settings in BIOS for 5810 or 7810, so assuming the same for R7910.

I have rack version of 7910.

May be  a change has occured on bios ?

Below are somme settings

Change Raid on to ACHI. That will solve the issue. 

this raid option is for sas controller, not for sata which is already set to ahci.