Hello, Amy.
I will assume that the extra Hard Disk Drive is used for storage, correct?
Follow the steps written below if that is the case:
1. Turn off computer and disconnect it from the power grid.
2. Take precautions against
Electro-static discharge3. Remove the computer casing
4. Handle the HDD with care when inserting it.
Now depending on the configuration on your PC:
If you plan on connecting the Hard Disk Drive to the same IDE channel as the HDD allready installed, set the new HDD to Slave and the HDD allready installed to Master.
The same applies to the second channel. However, the second channel (IDE 2) could be empty and you would need to set the drive to Master.
(Jumper instructions can be found on top of the HDD)
In case of a SATA HDD (Thin orange cable), you do not need to bother with Master/Slave or Cable Select as these are not present.
6. Once the Hard Disk Drive is fastened and connected to the Power Supply Unit and the Mainboard, it is safe to reconnect the computer to the power grid. You may wish to keep the case door open to see if no problems occur when booting the computer.
Windows should automatically detect the Hard Disk Drive. You may, however, need to partition it by going to:
1. Right click on My Computer
2. Manage
3. Storage
4. Disk management
I hope that this information is adequate, should you require more instance do not hesitate to inquire.