How do you securely erase data from your hard drive?

This can be achieved by writing a 1 bit for bit copy of zero across the entirety of your disk (just the sectors you wish to remove).

Their are several ways of doing this but one example is to use the Linux special device called "zero". Using the DD command on linux you can set the / dev/zero as the input file (if=) and your target disk as the output file (of=), this will proceed to write zeros until their is no space reaming on your disk.