Who ever told you that just connecting an external drive will get you a backup is an idiot.
If you will read this it will explainhow to use TimeMachine.
How do I back up informtion on my Macbook pro to an external hard drive? I was told all you need to do is plug in the hard drive to the computer, and it will back up automatically, which it is set to do. But all that is happening is the clock a the upper right of the page is revolving and the clock by my vault on the left is revolving, but there is no information as to mega or giga being copied.