for M2 to be bootable, there are few things required
- storage boot option needs to be UEFI only,
you can either disable CSM compatibility mode, but if you get black screen, keep CSM enabled, but switch storage to UEFI under CSM settings
- drive needs to have GPT partition, windows would tell you when selecting disk that drive cant be used if its in MBR partition scheme
- if its windows 7/8, you will need to add NVME driver during installation, otherwise drive wont show up in drive list, win10 has NVME driver already
if there is no operating system installed on that drive, than UEFI wont allow you to boot from it, this is normal
boot from flash drive to install windows, once installed, bios would show then windows boot manager as a boot option on your drive