reinstall on second hard drive -- product key issue

I tried to reinstall windows on a second hard drive, but it won't accept my product key, assumedly because I am already using it for the current windows installation on the first hard drive. How do I transfer my license to allow a reinstallation on the second hard drive? That's the best option, so I can go back to the first drive if I want to. Second option: If I wipe the current windows...