Hi [NAME] welcome to this free user-to-user Alienware laptop discussion forum.  The Crucial System Advisor database is a good source of information: Alienware m15 R7 | Memory RAM & SSD Upgrades | Crucial.com.  Support for Alienware m15 R7 Storage says M.2 slot(s) have PCIe Gen 4.0 x4 NVMe interface.  Please remove Base cover and check that second SSD is correctly installed within PCIe socket with good electrical connection. 
Hi, I recently installed and formatted a Samsung 980 PRO ssd in my M15 r7 second ssd slot.. However, it seems to be disappearing during certain tasks ex: moving files, installing games etc.. I am not sure what the cause of the issue is. From the forums and other online posts I read that others had issues with the same product. I have also seen people mention that the second SSD slot does not support gen 4 although it states in the manual that it does. There is a possibility of a faulty motherboard however, its very rare and to test I am thinking of purchasing another SSD which is known to be compatible with M15 r7 and doesn't really have any issues. I am not sure, if it needs to be gen3 or not. I don't mind as long as I can get a reliable SSD which is fully functional, installing games, storing data etc..  Please, if anyone can suggest some SSD options that are compatible it would be really helpful. P.s: I have my BIOS updated to the latest firmware and the SSD as well. 
m15 R7, second SSD disappearing and freezing