How Hard?
After a US spy plane was forced to land in China and the plane’s disks gave up military secrets, researchers worked to figure out how to quickly and securely erase the disk. Even when the people snooping it had unlimited time and money. Crushing, drilling or explosives would not do the job as fast and as safely as scientists hoped.

According to Dark Reading this proved possible only with permanent magnets – and not the kind decorating your refrigerator.

To create a magnetic field strong enough to penetrate the metal housing around a disk drive and erase the magnetic media inside, the researchers designed a neodymium iron-boron magnet with special pole pieces made of esoteric cobalt alloys. A motorized mechanism pushed disk drives past the magnets; a back up twist-knob allows operators to manually pull drives through the magnetic field.

How did they determine it really worked? By using the same techniques a serious disk recovery effort might use

The team verified that it was impossible to recover information from disk drives erased with the permanent magnets. They used a magnetic force microscope to map even the smallest magnetic domains on the surface of an erased disk drive to ensure that the patterns found there were completely random.

Disposing of disk drives that contained important information? Be even more paranoid than you are now.