Cloned a Windows installation from a SATA drive to an nVME SSD. As expected I got an error when booting from the nVME SSD "boot device not accessible".
Here's the solution:
Get to a menu where you can choose to boot the installation in Safe Mode (sometimes repeatedly pressing F8 or just letting the failure message take you to the menu).
Boot to Safe Mode.
Open a command prompt in Administrator Mode
sc.exe config stornvme start= boot
That command loads the nVME driver as default on the installation. Reboot.
All should be well and the Windows installation should now be able to boot normally off the nVME SSD.