The capacity of the HDD is the data volume index. The more Gigabytes the higher the capacity of the HDD. The average volume of a DivX film is 1.3Gb, the volume of an Mp3 music file is 10Mb (1/100th of a Gigabyte). You should select an HDD of the capacity compliant with your needs.
The external HDD is an electronic device with a certain capacity and power consumption. When you select an HDD you should take into account whether it will be used at places where a permanent power supply source is available (220V socket, an external power supply unit) or at places where the battery pack of the portable computer is the only power supply source (USB or battery pack power supply).
If you chose an HDD with USB power supply you should remember that it will consume the power of the portable computer, thus the battery life will be considerably shorter during the time when the disc is used.
The interface of the connection of the HDD is the connection with the computer, therefore when you buy an HDD you should verify that your computer has the relevant connection slot compatible with the HDD interface.
The convenient USB connection provides a possibility of connecting an external HDD to almost any modern computer and to handle the data stored therein both at work and at home.
Standard USB 2.0 connection:
During the next years the USB 2.0 will be replaced by the new USB 3.0 interface: