m15 R1, replacement PCIe 4TB SSD drive not working

I just bought a 4TB PCIe M2 SSD Drive from Crucial and tried loading it into the corresponding slot 1 and slot 2 connections (after removing the SATA one that came with it). So I should have 2 workable drives, but the laptop just keeps Blue-Screening. I put it back to the old config and it works fine. CMOS needs replacing, which is on order, but wouldn’t have thought that would have such an effect on it. Can’t seem to find how to change the priority of drive booting either. It only shows options for =

Any help appreciated.