@MartinWake What you said at the end of 3 would be correct; even though your BIOS says it's gonna boot from your main boot SSD, it is forcefully attempting to boot from your M.2 drive. So when you get the chance with your M.2 installed but before it attempts to boot into it (when pressing Del would open BIOS) press F11 instead. This opens the boot menu where you can manually select your SSD.