Unless I'm misreading this, it only mirrors your drive with no history. That's a poor approach - if your drive starts packing in you're likely to backup a fair amount of corruption before you realize what's going on.
Ditto if you screw up - you have to catch it before the next backup otherwise you're boned.
robocopy.exe /MIR <source> <target>
Create a scheduled task, set your external drive as the target, and forget. robocopy.exe is a built-in utility for Windows 7 and can be freely downloaded for via the resource pack. Also consider an offsite solution such as Windows Live Mesh for the free 5GB to create offsite backups of the most important folders. Never know when a catastrophic event (theft, fire) can take out your backups.