Thanks to the widespread adoption of IEEE1394 and USB2.0 interfaces and the current generation of inexpensive, large-capacity, fast access hard disks, many people are tempted to connect a 200 or 300 gigabyte external drive to their computers, and call it a backup.Read on over here.
I'm here to tell you, it just isn't so.
Why external HDDs aren't good as backup

Virtual-Hideout explains why external hard drives are supplemental storage, and not backup devices.