I think I've got it now

What I had:

(C:) SAMSUNG 980 SSD 500GB      PCle 3.0x4, NVMe M.2 2280   10.21   7.22

(D:) ST1000DM010-2EP102 Seagate Barracuda 1TB 7200RPM SATA HDD

(E:)  CD/DVD optical drive SATA

(F:) Crucial MX500 1TB 3D NAND SATA 2.5 Inch Internal SSD  9.2020

(S:) ADATA Swordfish 1TB 3D NAND PCIe Gen3x4 NVMe M.2 2280 Read  9.2020

(with S:) QNINE NVME PCIe Adapter, M.2 PCIe NVMe SSD M Key to PCI Express 3.0 x2 Expansion Card with Low Profile Bracket for PC Desktop

I proceeded to add a 2TB Crucial SATA Internal SSD, used Macrium to clone the existing F: drive (the main data drive) to it, then Disk Management to expand the volume to make the entire 2TB accessible.  Then I changed the drive letter so the 2TB is now the F: drive.  

The power cable in the 2.5" slot had two inputs, but not two data cables, so I ordered one.  That arrived today, and I installed it without any issues.  

What I have now:

(C:) SAMSUNG 980 SSD 500GB      PCle 3.0x4, NVMe M.2 2280   10.21   7.22

(D:) ST1000DM010-2EP102 Seagate Barracuda 1TB 7200RPM SATA HDD

(E:)  CD/DVD optical drive SATA

(F:) Crucial MX500 2TB 3D NAND SATA 2.5 Inch Internal SSD  9.2020

(G:) Crucial MX500 1TB 3D NAND SATA 2.5 Inch Internal SSD  9.2020

Missing:

(S:) ADATA Swordfish 1TB 3D NAND PCIe Gen3x4 NVMe M.2 2280 Read  9.2020

What I learned:  on the motherboard there are four SATA ports.  If there are fewer than four SATA drives, the NVME PCIe adapter will allow a stick SSD to use one of the ports.  When I added the fourth SATA, there was no more room for the adapter.  

Now the D: drive will be an onboard backup for everything on F: except the photos, which will go on G:.  There will continue to be multiple external backups, one of which will remain offsite.

One source of info, from Reddit, and another one, this one from Dell, Quote: "It has 4 sata ports (one blue two black one white).  If you have an optional ODD, then there are three left for HDD/SSD.  If no ODD, then 4 HDD/SSD max.

"note the white sata port is reserved for optional odd, but there is nothing stopping you from giving up odd and using the port for hdd/ssd.  as far as mobo is concerned, it does not matter whether it is odd or ssd.

"its 2.5 bay can accommodate two 2.5 hdd/ssd"