If your healthy hard drive drive has games on it, then it must already have at least one partition on it. If that partition fills the entire drive (ie there's no unallocated space on it) you can't create another partition on that drive.
However, if the existing partition does not fill the entire hard drive, you will be able to create another partition on it. I can't possibly say what size partition you can make, it depends on how much unallocated space there is.