Destroying data on broken hard drive?

A few weeks ago, I bought a new Western Digital external hard drive at Best Buy. It recently quit working all the symptoms indicate that its dead for good. (with the possible exception of professional data recovery) Fortunately I have everything that is important to me backed up. Obviously, since I spent over $100 to buy it just a few weeks ago, I’m not happy about the situation and intend to take it back to get an exchange. Unfortunately, the bad drive holds important confidential information that I’d rather nobody be able to recover and I’m not sure whether my files were completely destroyed when the drive went bad. Is there any way to make sure my data is destroyed without doing something that will void the warranty?

Know anybody with an extremely high powered magnet?