$60.00 for used $50.00 hard drive? Western Digital - My Passport Essential 500GB External USB 2.0 Hard Drive - Black - WDBAAA5000ABK-NESN
That might be wishful thinking?
Really, any of the programs already listed here will do what you want; however, whether or not any data is retrieved really isn't up to you. If someone really wanted to read that drive, they're going to read that drive. The only way to prevent that happening with certainty is to destroy the drive... hence the nail.
If you're giving it to some school or selling it to a friend, you're probably reasonably safe. If however you toss it up on ebay... well, who knows? I mean, who buys used hard drives? Mechanical drives only live a few years anyway. I'd be real suspicious of some guy stocking up on old drives.