Hi [NAME] harper , hmm, I think both slots will support m.2  PCIe NVMe SSDs according to dell but one can only support Gen3 and that is slot 2. Slot one can support Gen4 PCIe NVMe SSD.[NAME] guess this answers everything.  Got the answers from dell’s site and Chris.M  
Hi there, I'm planning to add another SSD to my m15 R6 (Samsung 980 Pro PCie 4.0 NVMe).  I read some posts about compatibility here in the forums but just want to be sure; Slot 1 can support Gen 4 SSD while Slot 2 can only support Gen 3 (correct me if I'm wrong) Currently, my Slot 1 has a Samsung Gen 3 PCIe NVMe SSD and Slot 2 is empty. I'm planning to move the SSD from Slot 1 to Slot 2 and install the new Gen 4 to Slot 1 so I can optimize the Gen 4 speed. My question is, after transferring my old ssd to slot 2 and installing the new ssd to slot 1, do I need to reinstall everything or is it just plug and play?  
m15 R6, transferring my SSD to a different slot