You are right to be concerned, having any backup in only one place is risky. An external hard drive can be dropped or stolen or stop working so dont have just one backup there.
I backup to 2 external hard drives AND a NAS drive. I also put important things like digital pictures on DVD.
There are a number of places you can make more backups:
On CD/DVD
On a memory stick (short term)
On a web based storage area (there are a number around, like Skydrive from Microsoft which is free)
On another computer.
You can use any or all of these.