Unfortunately, you'll need to grind the disks into microscopic pieces. From the same analysis I quoted earlier: To positively prevent data from recovery, disks can be removed from disk drives and broken up, or even ground to microscopic pieces. (Actually, simple disk bending is highly effective, particularly in emergency situations.) Obsolete government document DoD 5220 required physical destruction of the storage medium (the magnetic disks) for data classified higher than Secret. Even such physical destruction is not absolute if any remaining disk pieces are larger than a single 512-byte record block in size, about 1/125" in today's drives. As linear and track densities increases, the maximum allowable size of disk fragments become ever smaller.