“Pay particular attention to damaging the platters inside the hard drives where data is magnetically recorded” says UMich.edu.
Hard drives will typically have several platters mounted on the same spindle. See featured image. Be thorough.
Physical destruction is generally considered the most secure and permanent type of data destruction, but this must be done to a provable level, as even a small piece of a disk may contain data. Typical techniques include grinding, shredding, incineration, applying corrosive chemicals, or applying extremely high voltage.