|When introducing a new hard drive your BIOS may automatically configure that new drive as the first drive to boot to. You don't want that, so you need to verify the existing boot order, including which hard drive, stays as it is. |
Concerning what to do with the old drive, you won't be able to run any of the programs installed on it without first re-installing them. Your personal files may or may not be immediately accessible. You may need to take ownership of those files/folders. See if that is a problem before addressing it.