Unfortunately, computers (and particularly the hard disk drives that hold your files) can fail. Your computer might run for a decade with no problems, or it could fail tomorrow—there’s no way to tell. To make sure you don’t lose important files if your computer stops working, you should back up your computer on a regular basis. The backup process copies your files to a safe place so that even if your computer fails, you won’t lose them.
To back up your files to an external hard disk drive :
Step 1 : Start >>> All Program >>> Accessories >> System Tools >>> Backup
Step 2 : Back Restore wizard >>> click Next
Step 4: All information on this computer >>> Next
Step 5 : click Choose a place to save your backup >>> select your external hard disk drive >>> Next
Step 6 : Click Finish
Step 7 : Click Close