Even after random data has been written to the drive it maybe possible to recover data using special tools that government agencies have access to.

If this is a problem for you and you really think that the government is out to get you, then you should simply destroy the drive and buy a new one.

Exactly how you destroy it is up to you but I read that the US government has a system for destroying computer equipment by cross cut, crush, grind, burn and then spread on the roads as grit in winter.