That sounds hopelessly optimistic for any modern HDD and really sensitive data (though it would certainly stop the most common kind of snooper). Most of the surface of the disk will be intact and MOST of the data could be recovered. Even an intact chunk a few mm in size could contain a lot of data because the surface density of data is so high.
I think a cement kiln or maybe a hammer mill with a small enough sieve would be more effective and probably safer.
Even media that once contained top secret data is still considered top secret after being 'sanitized'.
formatting link