dd if=/dev/zero of=/dev/hda bs=1M

Adjust blocksize for performance reasons

Even better than all 0's, use if=/dev/urandom and run the command several times, it obfuscates the data on the disk even more beyond repair than plain zeroing


I personally think zeroing the disk is overkill in the first place though if all you're gonna do is reformat - unless something is malfunctioning, it doesn't matter if any data is lingering behind because it will not exist as far as the new filesystem from formatting will be concerned