So, Samsung Magician says this is disabled. Win 10 Pro. I have 1 Samsung Evo 850 SSD (OS drive) and storage evo 970 nvme m.2 (samples)
 EDIT: Also, PC boots fine and fast, BUT, I cannot set the OS drive, evo 850SSD as the boot device. It is NOT even an option. It is listed in the SATA list-It is SATA#1. But it does NOT appear in the boot menu? Very strange, to me anyway. I will google all this.
Do I enable it or leave it?
Thanks

I have an 850 EVO for the OS in Win 10 and I have AHCI selected. 
 

Going to give it a shot. Perhaps that is the reason I cannot select the 850 EVO ssd as the boot drive in the BIOS. even though it certainly DOES boot from the 850, I mean it is the only Win 10 installation.
Just saw your config. I also have an Asus Motherboard.
Thanks again
Frank

If your boot drive is not AHCI then changing it to AHCI in bios/uefi may result in a non-boot because the driver for AHCI is not automatically loaded with Windows. There is a registry hack that will allow you to do that after installation:
 
https://www.tenforums.com/tutorials/22631-enable-ahci-windows-8-windows-10-after-installation.html
 

OK Thanks. I will try and find out. It's an Asus Z370 Prime A

There is a big thread on the tenforums.com about the ASUS Z370 board.  May be most is about overclocking and tweaking but may be something to browse.
https://www.tenforums.com...us-x-motherboards.html

Trying to remember this, but the UEFI (BIOS) for ASUS is a bit unique, and the AHCI rings a bell. If you run the tweaking algorithm in UEFI was when it popped up on me saying that a RAID configuration is required (even if not used) or "x" feature gets disabled. I cannot for the life of me recall what "x" feature was, but wanted it. It should be in the documentation and may also pop up if you shift in/out of AHCI in the UEFI. To Win10, it doesn't seem to see a lot of what the UEFI has done, i.e., I am not using RAID in Windows.

Sheesh, so many acronyms while typing on a phone is just painful!

I have my BIOS at default, except for fan control for quiet operation. The last two BIOS updates I only changed the fan settings, the rest is at "factory default" as a BIOS update resets everything to "factory default".
The "ROG" motherboards have lots of options but I'm not overclocking so I leave it alone.
 

I am no overclocker either. That's why I saved a little $$ by getting the gen 8, i7 8700 and not the 8700K. 
I must say I have never , ever had a pc this quiet.
Noctua massive heatsink with 2 big fans and a Geforce GTX 1060 GAmi9ng x 6GB. It is actually an older one. 
Thanks guys
Frank

I have a "K" model but only because it was on sale at the same price as the non K model.  I have no need to overclock.