First let's check what you see in Disk Management. Right-click Computer in the start menu and select "Manage," then in the new window select Disk Management. That 1TB HDD should occupy one row across the page with partitions displayed as sub-sections. What sections do you see after C:? If it's unallocated space, just right-click C and select the option to Extend the partition, then let it take up the remainder of the drive. If there is another partition there, is there any data on it? If not, delete it and extend C:.