If you need to completely erase data from a hard drive, you need to check out the Drive eRazer Ultra (http://www.wiebetech.com/products/Drive_eRazer_Ultra.php) from CRU-DataPort.
It’s an US$249. standalone hardware device that erases all data from 3.5-inch or 2.5-inch IDE/PATA and SATA hard drives faster and more securely than software solutions. Deleting a file or reformatting a drive won’t permanently remove important information including credit card numbers, corporate and medical records, financial data, passwords or photos. Critical data remains accessible unless a proper tool is used to erase the hard drive entirely — and you’ll want to erase all that data before giving away, trading in, or selling your Mac when it’s time to get a new one.
Drive eRazer Ultra writes over the entire drive with new data, leaving nothing left to recover. To use, you simply connect a drive and follow the directions on the LCD menu. No computer is required.
An USB port is included so you can preview the drive and verify that it’s blank upon completion, which is very convenient. The Drive eRazer keeps up with the write speeds of modern hard drives and can operate at more than 7GB/min.