Well, if
you don't have access to thermite [...]
Actually, red heat is well above the Curie temperature for most media,
isn't it? You could chuck the platters into the coals of a bonfire,
let them get up to a nice cherry red. Depending on what the platters
are made of, this might even melt them, which should effectively
destroy any organization in the material that once formed a thin film
on them. If not, it should demagnetize the coating....
Unhealthy.
I would expect that to generate a fair amount of sulphuric and
hydrochloric acid.
A fair bit of the shit inside the HD also would end up in the nearby
soil.