If you have an operating system disk, like Windows, simply insert the CD in the drive and reboot. The operating system will reinstall and format the drive totally.
If you want to delete everything it's better to format your Computer. Choose the appropriate operating system as per your requirement. While formatting you should copy all your important files and documents in your external drive.
You need to "format" the harddrive. If you have the disc that is used to reinstall your operating system, this should five you the option to format the drive if you try to install it again.
