Using two known good Samsung 950 Pros (it's on the QVL), have tried them in each of the three M.2 slots. The BIOS always shows N/A for each slot. I read somewhere that GB MBs have a quirk where RAID mode has to be enabled in order to see M.2 drives. Just for laughs I did that and found that the RAID configuration recognized the drives, so that's perplexing. BIOS version is F4. Didn't want to upgrade that unless I had a specific reason.