If you want to securely erase a hard drive, it's not as easy as writing it over with zeroes. Sure, an "erased" hard drive like this won't boot up your computer if you just plug it in again. But if the drive falls into the hands of a specialist with a scanning tunneling microscope, they can tell the difference between "this was a 0, overwritten by a 0" and "this was a 1, overwritten by a 0".

As far as I know this has never been confirmed. See http://www.nber.org/sys-admin/overwritten-data-gutmann.html for more details.