How to erase hard drives (shred, dd)

You could write zeros to the whole hard drive

dd if=/dev/zero of=/dev/hda

For newer versions of dd, you can monitor the progress:

dd if=/dev/zero of=/dev/hda status=progress

However, it is better to write a few passes of random noise (n = 3 is the default)

shred -vfz -n3 /dev/hda