I do have a recycle service that will shred the hard drive.
"All material destined for recycling, including hard-drives, is shredded in our
low speed, high torque shredder. All materials are shredded to under 2". A
cross belt magnet recovers all ferrous metals. Aluminum is removed from mix
by eddy current; and the residual material is sent to an end destination, either a
lead or precious metals smelter, depending on components. "

So there are pople who know how to do this, allow you (for a fee) to witness the destruction and sign off that it was done.