The only proper way of disposal is physical destruction, preferrably on site and under observation of inhouse staff. There are contractors you can hire in those: https://shredsupply.com/hard-drive-shredding-trucks/ (no relation, just arbitrary google hit).

I'm also unsure as to why you are getting drives back at all, any of your customers should not have any kinds of storage devices leave their site intact. At least that is the standard over here in Europe for healthcare and other industries dealing with sensitive data.