External hard drives offer you the flexibility to back up your files to free up space on your computer or to save a mirror image in case your computer crashes. Some must be plugged into a wall electrical outlet for power, but others can draw power from the computer using USB cables. If your hard drive's cable has two USB plugs on one end, it means the drive doesn't need a wall outlet to work.

Types of Hard Drive Power

External hard drives don't have internal batteries that hold a charge. Instead, they rely on outside sources to provide power when you need to activate them. For larger, more powerful hard drives, this often means using a wall outlet. Smaller, portable hard drives often need only a USB cable to transfer power as well as data between the computer and the hard drive. This type of drive is ideal for people who stay on the go and need to back up their laptops regularly.

Why Two Plugs

Hard drives sometimes need more power than they can draw from your computer using a single USB cable. In this case, the hard drive typically comes with a USB cable that has a standard connection on one end and a Y-shaped split cable on the other, each ending in a USB connection. Power flows through the main section of the Y-cable along with data. The second connection is solely for power, not for use transferring data.

Using the Y-Cable

The external hard drive might not require the use of the second USB plug every time, depending on how much data you're transferring and how much power is released by the computer connected to the external hard drive. Attach the section of the Y-cable marked "Data Plus Power" to a USB port on your computer first; if the hard drive doesn't turn on, plug in the second part of the cable noted as "Power Only." If your computer doesn't have enough free USB ports to support two from a Y-cable, connect a powered USB hub to your computer and connect both Y-cable ends to the hub.

A Note About USB Connections

Although many existing computers and external hard drives use USB 2.0 technology, newer models integrate the USB 3.0 connections, released in 2008. The 3.0 standard enables faster data and more power to flow through the cable, as long as the cable and both ports -- on the computer and the hard drive -- are 3.0 versions. This means the hard drive is unlikely to need a Y-cable to transfer data as long as both ports and the cable use 3.0 technology; data and power transfers are restricted by the slowest port or cable, so if one is 2.0, the hard drive might need additional power from a Y-cable or wall outlet.