There are techniques used to recover previously erased information on HDDs. These techniques have been available since the 90's. The best way to make it unrecoverable is to melt the disks by the use of whatever techniques you have available, including ox-acetylene torches (the favoured method for the company at the time I was working for them), putting into a furnace (as suggest by others here), etc. The problem with…
These techniques are theoretical from the time HDDs needed a user low level format(think RLL controller in your XT), and even then a myth nobody ever demonstrated in practice.