mettelus
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!

That's exactly what I see, the pop up about the raid config is required or X will be disabled crap. 
I just get not get my head around the ffact that I can boot fine and very, very fast, yet my OS\boot drive is NOT listed in the boot priority section where I can change the order. My optival drive and my nvme m.2 drive are the only ones listed? Go figure.
 
Think I'll search the Asus MB sites as FB suggested.

fireberd
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.

 
I had no intention of overclocking either. But when I initially booted up my new system I found the CPU freq to be set 4.1GHz rather than the base freq 3.6GHz. I had purchased an ex-demo motherboard and presumably it had retained the settings from its previous use. This has never caused any apparent problems so I just left it as it was, i.e. overclocked.

Win10 plays a part in things too. I was initially wondering why the disk numbers were all out of whack (they are actually in exact reverse order of the letters), then realized it would take way to much effort to change them so just let them be. It just looks really weird in imaging programs when "Disk 3" is the C: drive. If your machine is working without issues, don't get too caught up in trying to micromanage things. One feature I do appreciate with the new boards is that they will update the BIOS via the internet without issues.

Speed may not be the issue with AHCI, but at least in earlier Windows versions, AHCI was reportedly needed (or not) to use Windows TRIM. If that is true of Win 10, then you should enable AHCI. My Prime 370-A came out of the box with AHCI set, so unless you have changed it, your host controller should be able to use it. To see if Windows has loaded an AHCI driver, look in device manager under ATA/ATAPI controllers for an AHCI controller. If none is there, you will need to set it in uefi, with the caveat mentioned in my post above. 
 
To see if TRIM is enabled you need to open an elevated command prompt (CMD in search box to bring up Command Prompt icon, right click on that and choose run as administrator from the drop down) and type:
fsutil behavior query DisableDeleteNotify
hit return and if it returns:
DisableDeleteNotify = 0
then TRIM is enabled.
 
Google should have brought you to a number of reports from people who have your drive who have had similar problems and seem to have resolved it by using a mysterious ritual as follows:
set an open Windows session to boot at the next boot so that it boots into safe mode
restart
enter uefi prior to the loading of Windows safe mode and set your controller to AHCI
close UEFI and allow to fully boot into safe mode
restart 
 
I have no explanation for why that might work, except that there may be an installed driver that is hijacking Windows recognition of your drive and plugging it into the AHCI controller driver. Safe mode may bypass that and let Windows do its thing.
 
 

Hey SB
 
Just saw this. Great info. I did find many posts like you stated. I did finally get it changed but still my Samsung evo 850 ssd os\boot drive does NOT appear in the boot priority but does work fine and does appear in the boot menu F8. So, as suggested here, I will not worry about that .
I did s you suggested with device mgr and I do have 2 Achi drivers listed in Device mgr-one standard and one intel 300 series chipset achi.
Also, yes trim is enabled-thanks for the command prompt....
Is their anyway I can tell which one my ps drive is using? Does it matter?
 
Thanks for the great info!

If you change the HD controller mode, boot into Safe Mode... then restart the machine.
That should solve the issue.
 
If your new SSD doesn't show in the boot order, go to the HDD BBS Priority menu... and make the new SSD the first drive in that list.  It should now show up in the boot order.  

Thanks Jim!  That did it!