WOW, much basic, but not intuitive in the slightest.

Trying to install Win 10 Pro to M.2 drive. System/MB supports the OS, but noting the difference in steps whether the M.2 drive is seen in BIOS, boot is on UEFI/Legacy/Both, and the NVMe Firmware Source setting for SMC systems.

BIOS Optimized defaults

Non-UEFI USB Boot Media installation - selecting M.2 drive to install Win10 Pro

Stops here.

UEFI USB Boot Media installation - selecting M.2 drive to install Win10 Pro

Works.

After it finishes installation, it warns that it is automatically rebooting. Windows Boot Manager and the M.2/NVMe drive shows up at the top of Boot priority.

On default settings, it will not list the M.2/NVMe drive as a bootable disk, but it will be seen in the Win10 Pro installer. After installing, it will show up as 'Windows Boot Manager' with the full part number of the M.2/NVMe drive.

After selecting the Windows Boot Manager right after the Win10 Pro installation rebooted the system, it will run a disk check and repair that will look like below screen.

This takes about 3-4 minutes, and will reboot once more. Without interrupting system boot to visit BIOS Menu, it will boot straight into Windows 10 Pro Basics and Welcome screens.

Language, Keyboard Layout (Skip). It did not list any available Network ports, so I used 'I don't have internet'.

After Account, Password, and declining all services, it will take seconds to finally bring me to a desktop.