If BIOS (UEFI) is still set to RAID, you may also have to change that to resolve the problem, in addition to initializing the old SSD (ALL FILES DELETED!). You can't just open BIOS setup and make the change. You have to prepare Win 10 for the change. Otherwise, you'll end up having to do a clean install...  
XPS 8940 here, come with a Western digital PC SN520 NVMe (256GB). Bought a Samsung 970 EVO Plus 1TB a few months after, removed the WD and installed the 970, no sweat. I kept the WD for a few months in case the 970 would develop an issue, which did not happen. So decided to reinstall the WD on a PCIe expansion card (M.2 NVME to PCI-E 3.0 4X with M key). I tried to install but the desktop tries to boot a few times  then tell me there is a problem with the OS. I guess it tries to boot from the WD on the expansion card instead of the 970. Looked at the BIOS setting and 970 seems to be first in line in booting sequence. By the way I bought a second expansion card thinking the first one was defective, same issue. What is the best way to configure my BIOS, unless there is a limitation or issue preventing me to do what I want to do ! Thanks in advance.
XPS 8940, NVMe M.2 drive on PCIe 4x expansion card, does not work