I'm not sure whether you mean you're adding a second hard drive or you're replacing your current hard drive?
If you replaced your hard drive then you need to set the jumper to the correct terminals on the hard drive for it to work properly as a primary drive.
You then may have to go into settings on the BIOS and set the computer to boot from that hard drive.
If you're adding a second hard drive to your computer and it's not recognizing it then you
Go to Disk Management.
Then Find your second hard disk drive, right-click on it and go to Change Drive Letter and Paths.
Then your computer should recognize the second hard drive.