How to use product key for second hard drive

[Guest]

So I've recently purchased a Windows 10 product key for rather cheap, and I can't figure out how to use it.

I wanted to be able to have both Windows 8 and Windows 10 on my PC, since I have multiple hard drives.

How would I go about this?

 

[USAFRet]

Why both?

(I won't comment on "rather cheap" and "likely to become invalidated")

 

[Guest]

Compatibility. I've read elsewhere that some games just don't function well on Windows 10.

Also, I bought it from G2A? Is that...bad?

 

[USAFRet]

Compatibility. I've read elsewhere that some games just don't function well on Windows 10.

Also, I bought it from G2A? Is that...bad?

That is not a valid source for licensed software, and licenses from there have been known to become invalidated.
http://www.polygon.com/2015/2/9/8006693/the-truth-behind-those-mysteriously-cheap-gray-market-game-codes


And which games have you "read about" not working with 10?