Are you wanting to use the old hard drive as a backup drive? Or are you wanting to use it to run your system with the OS already installed on the old drive?
If you are wanting to use the old OS, then you will need to reformat and reinstall the OS. Trying to use a copy of a OS out of a different system leads to tons of problems. It is always best to reformat and reinstall.
If you are wanting to use it as a back up drive. Then once you have your new drive up and running with the OS installed, you can then install the old drive, and pull any files you want off of it to your new drive and then reformat the old drive.