Removing all files on hard disk without installing new system

Hello Tech Republic,

This is my first thread here and I want to say hello to everybody on these forums.

I have 2 hard disks which one of them is from my old PC which has Windows 7 on it. Other one is the one I’m using on my PC.
So the problem is, I’m running out of space on my main PC but I don’t want to install new system. When I plug in both hard disks at the same time, I can’t run PC because there are system files on both hard disks.
My question is, is there a way to remove all files from old hard disk so I can use it on my main PC, or do I have to install new system all together?

Thanks in advance.