Like TURBO said, "Just instal the old HDD in a different machine."
I'm assuming you'll be getting another PC? The data on the old HDD will be fine, you hope. j/k.
When you get another machine that you'll be using, you can install the old HDD in it. For more information how to do this, post back on TF when you are ready and someone will walk you through it.
I'd advise a FULL (virus, adware, spyware) scan of that old drive before you copy or execute any files on it. Then once it's clean, transfer any data you want to the new HDD and format it. Then you'll have a another drive for additional storage. Or take it out if you want to.