just dban or secure erase. Every week we get people asking this question and a thousand people giving crazy ideas like put it in microwave or pour cup of water on it. Just wipe the damn thing with DBAN or Secure Erase. Preferably Secure Erase followed by DBAN. Secure Erase is best. DBAN is second best. Both provides defense in depth in case of implementation errors in either. Nothing else is likely to be as secure short of melting the thing down into a liquid.