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 erasure is that there is residual amounts of magnetic fields that can be picked up by the right equipment. In the day, it was reported that they had success with recovery of information that had been overwritten by 8 - 10 times. It just took a lot of patience to do so. Those who want to recover this information will have that patience.
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.
The problem with erasure is that there is residual amounts of magnetic fields that can be picked up by the right equipment. In the day, it was reported that they had success with recovery of information that had been overwritten by 8 - 10 times. It just took a lot of patience to do so. Those who want to recover this information will have that patience.