If you are adding a second hard drive, the easiest option is an external unit. These use either USB or Firewire interfaces that plug into the computer tower. These hard drives install very easily and don't require you to open the case or mess with jumpers. In addition, you can take your data with you if necessary. However, external drives lose some of their potential performance by not being connected to the faster internal interface. In addition, you usually can't boot from these drives on Windows systems. Most external drives need top be plugged into an electrical outlet, though some portable drives use USB power to operate. Of course, these portable units actually contain slower laptop drives. They are good for times when you need to use them somewhere that AC power isn't available. Just be aware that such a device will actually be using your laptop to power it and will drain its battery faster.

If you are replacing a system drive or building a new system, you'll be wanting an internal drive. If your motherboard has the proper support, you currently have two choices: Parallel Advanced Technology Attachment (PATA) and Serial Advanced Technology Attachment (SATA). Most motherboards support PATA drives, while newer boards can support SATA, which provides improved performance over PATA.

In the case of desktop systems, you'll be looking for 3.5" drives, which are the current standard size. These drives can be quite fast and support sizes in excess of 500GB.

For laptops, you'll be using a 2.5" drive.These devices are slower and have smaller storage capacities. They also have smaller connectors, so they won't directly work in a desktop system. These usually don't require any jumpers. Be careful when connecting or disconnecting them as it's easy to bend the data pins.