Sorry you are still having this problem. Maybe RedSector3 was onto something with the suggestion that the old 960 and new 970 SSD are set up differently perhaps with respect to legacy booting (with MBR partition) versus UEFI booting (with GPT partition). If your BIOS is set up for the more modern UEFI booting, but the 970 somehow got cloned with an MBR partition for legacy booting, then it won't show up in the boot priority list within UEFI BIOS. Usually in UEFI the boot device shows up as "Windows Boot Manager" or "UEFI: Samsung 970 Pro" or similar rather than just the hardware name. Also you may know that for UEFI booting you have to disable CSM (Compatibility Support Module) in the BIOS as that is used for legacy boot devices that may be partitioned MBR. You could go into BIOS and verify that you are using UEFI booting, and also verify that the new 970 is using GPT partitioning. If both true, then make sure CSM is disabled. See if you can find "UEFI: Samsung 970 Pro" or similar in the boot list.
If everything looks right in terms of using UEFI booting, you could boot using F8 (the 970 SSD, for example) and go into the Windows Disk Management Console as Admin. Your boot drive should be in the list. Say the Windows partition has drive letter "C", then you should see something like this:
One of the parameters for the "C" partition should be "boot". If not, right click and see if you can mark the parttion as "active". Have everything backed up of course.