What Cindori said.
Also, though it might be a bit of overkill, one possible option would be to boot a Live Linux from CD/DVD (booting non OS X Systems from USB seems to be tricky) and overwrite the drive with zeroes.
I've tried this with a Xubuntu 9 i had lying around, but I guess you could do it with any recent distro. You can get one
here. What you wanna do is remove all hard disks from your system except the one you want to erase. Then boot your Mac into Linux (press Option key at boot time, select 'Windows' from CD, then 'Try Linux without installing') and open up a terminal (most likely you'll find one in Applications->Accessories->Terminal). Then you enter:
which should give you something like this:
Code:
/dev/sda /dev/sda1 /dev/sda2
sda is the whole disk, sda1 and sda2 are the single partitions. Since you've removed all other hard disks, there should be only one whole disk listed. Note: USB media would show up here as well.
!!!You definitely want to make sure that you've got the right disk, because next we're gonna overwrite every single byte on it!!!
And that's how:
Code:
sudo -s
dd if=/dev/zero of=/dev/sda
This could very well take a couple of hours to complete. After it's done, your hard disk is full of 0s, which should be enough for keeping some hypothetical sleazy guy at WD watching your family videos. (Brrr, creepy thought.)
Please use this info at your own discretion, i have to stress it once more: this erases one complete drive. Afterwards, everything on it will be gone.
Good luck.