Hello, I'm basically having the exact same problem as referenced in this thread : http://en.community.dell.com/support-forums/laptop/f/3518/t/20021480 

I received the laptop yesterday. It came with the 256gb Toshiba NVME SSD and an empty SATA slot. After updating windows and installing the latest drivers from the dell driver support page (included the latest bios, version 1.1.3) I turned off the machine and installed a Samsung 850 EVO 500GB SSD in the spare SATA slot. As in the referenced post when I turned the laptop back on the hard drive was not seen by the BIOS, Intel RST, or Disk Management within Windows. The BIOS is set at it's defaults which are UEFI and RAID I believe. I went through the Dell Diagnostic and no issues were found, the laptop performs perfectly except for this issue.

I'm happy to go through troubleshooting but in the linked post user ttmcmurry has really went all out documenting the issue so I'm not sure what I can add on that would be helpful.