Bios no longer recognizes the NVMe drive once you removed it and then reinstalled it.
Bios no longer recognizes the NVMe drive once you removed it and then reinstalled it.
Have you activated Fast Boot?
According to the manual for the Gigabyte Z270 Gaming K3 (chosen randomly without knowing your exact motherboard), there is an option called "Last Boot HDD Only" under SATA support. This setting turns off all SATA devices except the previous boot drive until the operating system completes its startup. It seems to work mainly with SATA and not NVMe, but it might still help.
It could be that previously you used a 500 GB SATA drive and booted from it; later you upgraded to a 2 TB NVMe, installed Windows, and now the system thinks that old drive is the previous boot device. If you removed the old drive, installed Windows on someone else's NVMe, and then put your own NVMe in without installing Windows, the system might now recognize the 500 GB SATA as the previous boot device instead.
Even if this isn't the case, disabling Fast Boot or re-enabling it could resolve the problem.
Configure BIOS to start in UEFI only (or UEFI first).
Select the initial boot device as Windows Boot Manager on the 2TB storage.
Also turn off Fast Boot.
If CSM is active and legacy boot attempts first, your system will boot from another drive where Windows is installed in legacy mode.
Additionally, please specify the exact Gigabyte Z270 model board you are using.
On certain boards, using a M.2 drive might conflict with some SATA ports.
Refer to the motherboard user manual for details.
I took the lesson to heart and won't repeat it. I transferred all my files from the hard drive, reinstalled everything, and then restored it. Everything is now working properly. It only took me a day to complete the process. Thanks for all the suggestions. It was quite frustrating...