I got the 970, and I did the reinstall (from USB). I did NOT unplug my SATA HDD.

Once I reinstalled Windows, it would ask which Windows I wanted to boot every time. So I googled and found instructions to choose the default Windows to use. Sorry, don't remember the details but it was pretty straightforward, and since it's not something I expected to do again I didn't bother filing it away :).

I then deleted Windows files on the old drive, cause I didn't intend to ever boot from it again and I like having orderly files.

My funny confusing bit was I saved my Windows type accessed stuff like documents and downloads, etc on my OneDrive. Once I did that Windows decided to use them instead of my local ones (I'd copied them over) and then I had a strange icon on all my desktop icons (everything worked fine). Took me days to work out what was going on lol. But I just reassigned them to local and it was fine.