External hard disk drive are the drives which remain outside the CPU. They are connected to the system via USB cable. The internal components of an external hard disk drive are similar to the hard drives which are found inside the system. They also have the platters and moving heads like in the traditional hard disk drives. They are found in different sizes like the 2.5 inch and 3.5 inch drives. They also support plug and play, so we have to just connect the USB cable to the USB port of the system. And the system automatically recognises the external hard drive as a storage device and asigns a drive letter to it. In some cases we will have to install the device drivers which comes with the external hard drive package.