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@john78oneill  If you can't see it in Windows, go into your BIOS Setup and look for the System Information/Summary section.  See if it's listed even there.  If not, then it's not even being detected at a hardware/firmware level.  My first test would be to reseat it.  I know it seems unlikely that that could be an issue, but for example I've seen multiple people report that their Samsung PCIe x4 SSDs only negotiate at x2 or even x1 speeds, and in every single case where I've suggested reseating it, that solved that issue.  Apparently the proper connectivity tolerances on that connector are narrower than one might think.

If that doesn't work either, then you'd have to start isolating variables, i.e. testing that SSD in the other slot or testing your known good SSD in that second slot.  Again, you just have to boot into your BIOS Setup to see if your system detects the SSD at all.

As a last resort, try updating the firmware of your system and (using another system) the new SSD in case it's a weird interoperability issue.  I remember that the XPS 13 9350 and XPS 15 9550 for example didn't work with Samsung 970 SSDs, even though they worked fine with the 960 models.  No underlying cause had been identified last time I checked, and it may have since been fixed with firmware updates, but if not, then this could be an issue like that.  Unfortunately not everything in the tech world that SHOULD work always DOES work.

The other potential issue is that these systems ship in RAID mode by default.  If you don't see the drive, open the IRST utility under Windows and check there.

You may need to set up the drive with the RAID utility before you can use it.  On many Intel systems, CTRL+I will trip the RAID setup before the OS starts loading.

@ejn63Thank you for the response! Where can I find the IRST utility? I don't currently have it and while looking for it online, I couldn't find where to download it.

I tried Ctrl+I during boot, it didn't do anything.

@jphughanThank you for the response! In my BIOS Setup, I can't find a System Information/Summary section. Does it have another name?

@john78oneill  It used to be called one of those things, but Dell may have redesigned the BIOS Setup interface for new systems.  But somewhere you should be able to find a page that summarizes the system hardware, i.e. CPU, memory, and storage devices.

I don't think RAID mode is the answer here.  Even if the system is in RAID mode, Intel's RST controller "passes through" individual disks that aren't part of a RAID virtual disk, so that wouldn't prevent your storage device from being visible.

@jphughanthanks for the explanation. Under Storage, I found a list of the drive slots and what's in them (if anything). The second M.2 PCIe slot shows as empty.

I've reseated it twice, still nothing. If I try to reseat it any more, I'll completely strip the screw and be even more than I already am (looks like something else I need to buy, hooray).

I'll try testing the SSDs in different slots to see if it's a bad SSD or a bad slot. Neither option is fun, but c'est la vie.

Thanks for your help!

@jphughanI'll look into those. Thank you!

@john78oneill  Sorry I don't have a better answer for you here.  But for what it's worth, fastening screws for M.2 SSDs are usually M2x3, although that isn't mandatory.  If you stripped the screw, then you might be using a screwdriver that's a bit too large.  A while ago I bought this kit, which is absolutely fantastic.  Even if you don't do MUCH work inside electronics, I feel that the avoided time waste and aggravation for even a small number of jobs is well worth the cost of the kit.  I have since upgraded to this kit, which includes that driver set and some other useful gadgets.  It's more expensive, but the additional tools can make it a LOT easier to pry open devices that rely on clip tension to stay closed (like the XPS 15....) without damaging either the chassis or an internal component.  And some other included tools make it a lot easier to disconnect tiny connectors without accidentally pulling a wire out of them.

*Disclaimer* I don't actually know the specs of the board and I'm just pointing something out which may not be the issue at all.

Just a heads up that there is a few issues here with M.2 slots.

- M.2 is just a type of connector but it can support different standards.  Other than viewing board specs there's no way to know if it's msata or PCIe. 

- Even if it is PCIe it may not support NVME

- Also even if it does support PCIe and NVMe there needs to be enough PCIe Lanes to support another NVME ssd.

- lastly for whatever reason it seems like some mobo's just don't like specific SSD's...