dd if=/dev/zero of=/dev/[drive]
If you do go about wiping a drive to get rid of sensetive data, you don't want to write zero's to the disk. You want random data. Instead of /dev/zero, you would use /dev/random . If it were zeros or a predictable pattern, the old data can be recovered/stolen. Also you would want to do it several times.
dd if=/dev/zero of=/dev/[drive] && dd if=/dev/zero of=/dev/[drive] && dd if=/dev/zero of=/dev/[drive] && ....
You get the idea. I usually 3 times over will keep anybody from recovering your data, but at one point I read that the CIA/NSA types could recover something overwritten as much as 17 times, which is mind boggling.