You may want to run the PC hard for a few days, just to make sure everything is working properly.

Then you probably should change BIOS from RAID to AHCI first. It has to be done the right way or PC may become unbootable:

1. At the desktop, open CMD prompt window, run as administrator
2. Copy-paste this command, which will start Windows in Safe Mode the next time you reboot: bcdedit /set {current} safeboot minimal and press Enter
3. Restart the computer and tap F2 when you see the Dell screen to enter BIOS setup.
4. Change SATA Operation from RAID to AHCI.
5. Save the change and exit Setup . Windows will automatically boot in Safe Mode.
6. Open CMD again, as in step #1.
7. Copy-paste this command, which will start Windows in Normal Mode the next time you reboot: bcdedit /deletevalue {current} safeboot and press Enter
8. Reboot and Windows will automatically start with AHCI drivers enabled.

Now you can install the other SSD. NOTE: If Dell didn't provide a M2x3 screw for the (empty) NVME slot, you'll need to get one before you install that SSD.

If that other SSD has Windows on it now, you'll have use the F12 menu to make sure PC boots from it's OEM SSD. Then open Windows Disk Management to initialize that other SSD for storage. ALL FILES WILL BE DELETED so make sure you select the right SSD to be initialized, and any files you need on that drive are backed up elsewhere.  Then reboot normally and other SSD should be available for storage.

If that other SSD doesn't have Windows on it, you don't need to run Disk Management (unless you want to change its drive letter). Windows should recognize it just by booting normally rather than via F12, and should be able to read/write to it.

@RoHe​ 

Hello Ron,

Thank you for the quick response! The other SSD is brand new so I believe that means it would not have Windows on it. It is a Samsung 980 PRO PCle 4.0 NVME M.2 SSD 2 TB.

So that means do steps 1 through 8, install the drive, turn the PC back on and it should be good to go?

Yes, do #1-8, especially because it's a Samsung SSD. Samsung warns their SSDs and software don't play nice when BIOS is set to RAID.

Since it's a new SSD, after it's installed (don't forget to install an M2x3 screw), you have to boot to desktop, then open Windows Disk Management and initialize the Samsung SSD.  Otherwise, Windows won't assign a drive letter and won't use it.

And if you're only going to use it for routine storage, rather than backup storage, you will want to change the location for Windows default folders so they're stored on the new SSD (or an internal HDD, if you have one), rather than on the boot SSD, C:

@RoHe​ Followed Dell Service Manual to add SSD which does not mention change to AHCI (only had original 1TB SSD). Now get BSOD. Removed 2nd SSD but still getting BSOD (stop code Inaccessible Boot Device). The new SSD (for extra storage only) is Crucial T500 1TB. What can be done to get running again and then follow your suggestions?

Sorry to hear that, but I did warn twice to change BIOS to AHCI, as above, before you installed the new SSD, or PC would become unbootable. 

Is BIOS still set to RAID? If not, make it so.

Create a bootable Win 11 USB using Microsoft's Media Creation Tool.  With new SSD removed, plug that USB into XPS 8960 with power fully off.  Then power on and tap F12 when you see the Dell screen. Select USB boot option on the F12 menu.

When the Windows installer loads, select the Troubleshoot option and then navigate to Startup Repair.  If you're lucky, it can fix the problem. Then make sure PC boots normally and everything works, and then change BIOS from RAID to AHCI before going any further.

If not -hopefully- you have a very recent backup image of the boot drive...

@EdMXPS8960 - Please don't double-post the same question...

@GWGDell​  How did you do following the procedure detailed by RoHe?

@RoHe​ Ron,

will this procedure void the warranty? I’ve been told that opening the case will make the warranty void?

thanks

@Sailpilot0253​ - Don't know who told you that nonsense, but opening the case does not void your warranty. 

If you replace an OEM drive or just install an additional drive, add more RAM, etc and then have hardware problems, Dell can ask you to undo your hardware modifications and revert back to the factory config before they'll offer warranty hardware coverage.

And obviously, Dell's warranty doesn't cover any component you installed, if that part fails...

@RoHe​