Cannot get rid of old HDD

Hello, I'm CrazyKilla Firstly I'm sorry or the bad title, but I could not summarise it in any other way. I have 2 HDDs in my build at the moment, a old 150gb one and a new 2tb one. I have windows installed on the old one, and on the new one; The new HDD is split into two, X: and Z:, Z is for windows and programs and X is for games and other crap. My old HDD is C:, this is where I had...