According to Pfitzner R., 2003, at least 33 overwrite passes with pseudo-random data are required to irrecoverably destroy the contents of a hard disk.
There are several different overwriting patterns, proposed by various intelligence, the military and government organisations. It is unlikely that a government publishes a method that will protect you from its law enforcement and intelligence agencies at the date of publication. E.g. the DoD 5220.22-M pattern (3 passes) is not approved for sanitizing media that contains secret or top-secret information by the DoD itself.