Some computer users like to run two hard drives on one machine, so they can keep certain functions separate. For instance, you might want to use one hard drive for business and the other for gaming, so the files and programs for each purpose will not interfere with one another. The documentation that came with your computer or motherboard can tell you which type of hard drive--Serial ATA (SATA) or integrated drive electronics (IDE)--to purchase. Once you purchase a second hard drive, installing it in your computer will require a little computer surgery, but nothing too complicated.
Items you will need
Open your computer case. Usually, you will have to remove a couple of screws from the backside of the case, then slide off the side of the case.
Look for a free Serial ATA (SATA) or Integrated Drive Electronics (IDE) slot on the motherboard and a free cable coming from the power supply identical to the one attached to your existing hard drive. There should also be a free slot in the metal bracket system holding the first hard drive and other disk drives in place in the front of the computer.
Insert the new hard drive into the free slot in the brackets in the front of your computer case. Attach the screws that came with it to make sure it is firmly in place.
Connect the power cable and motherboard cables to the back of the hard drive and replace the side of the case. Turn your computer on, and your new hard drive should be ready to use, though your computer might ask you to install drivers for it. If it does, you will find the drivers on the disk that came with the drive.