I had quite an education on installing Windows 10 on an Asus ROG Maximus Hero X Motherboard.
I tried over 40 times, and I am, well I thought I was good at this….
1) Remove all Disks (SSD, HDD, and M.2) except the one the you are installing Windows onto
2) Set SATA to Intel RAID if you plan to use it on completed machine, or if your installing on a RAID Partition. Remember that you need to load the Intel RAID Drivers when installing Windows if you using RAID, so have them on a USB.
3) CMS set to DISABLED, if you use AUTO, the install (for me on a SSD on the SATA Ports), it does not install as a UEFI
4) Set to a Non Secure, other OS. If the grayed out part shows ‘Secure Boot’ then erase the security keys. You can back them up as well.
With only one strange device, CMS Disabled, no Secure Boot and SATA set to your desire, install Windows 10.