The best way to destroy a hard drive is to magnetically degauss it first. If a hard drive is passed through a very strong magnetic field, the data on it will be permanently destroyed. The platters that spin the drive should then be physically destroyed.
The hard drive can be physically destroyed by pulling apart the platters. Open the hard drive using whatever tools are available. The platters must then be scrubbed by using sandpaper. Next, break the hard drive into bits by using brute force. Dismantling the hard drive and making it completely useless is a time-consuming method but it works best.