A hard drive shredder is a mechanical device that physically destroys old hard drives in such a way that the data they contain cannot be recovered. At a minimum, this involves severing every track on every platter in a drive.
The data bits on a hard drive are not destroyed by formatting, or even by overwriting several times with other data. Specialized programs called hard drive shredding utilities overwrite data many times with "gibberish," but some engineers claim that the original data may be recoverable by a determined expert even after this process unless overwriting has taken place hundreds of times. Drilling holes in an old hard drive will not necessarily work, because some tracks are likely to remain unsevered unless hundreds of holes are drilled.
The most effective way to ensure complete data destruction on a defunct hard drive is to pulverize the assembly, in much the same way as a wood chipper disposes of tree limbs and brush. Environmental precautions must be taken when this is done, because the resulting particles can be toxic. Several companies offer hard drive shredding services for a fee, including secure pickup and transport, compliance with environmental regulations, and certification of data destruction. For the enterprise concerned about data theft, such a vendor may offer the best solution to the data destruction problem.