How Do I Securely Erase My Hard Drive?
Users of Mac OS X 10.6 Snow Leopard should insert the Mac OS X Install DVD and then restart while holding down the "C" key. Select a language, press the right arrow key, then under the Utilities menu select Disk Utility. Choose your hard drive from the sidebar and then click the Erase tab. Click the Security Options and you can choose to "Zero Out Data", "7-Pass Erase", or "35-Pass Erase". These options will write zeroes over your entire disk - the more passes you take of zeroes, the more securely the data will be erased.