You hear three thuds on the door, followed by the words "open up, FBI!"
The first thing that runs through your mind, after a few obscenities, is that you've got hundreds of gigabytes of illegally-downloaded movies and music on your hard drive, and dozens of saved e-mails from an associate in China who leaves large sums of cash for you inside a safe-deposit box in a nearby bank, and a bunch of work you've done that exposes a backdoor the NSA put into AES [pdf].
What to do, what to do? If all of these things were on CDs, tossing them in the microwave and letting 'em fry for a few seconds would do the trick. But, alas, hard drives are harder to kill than CDs. These days, data recovery labs can extract data from even the most mangled hard drives (albeit for a price, but Uncle Sam certainly isn't short of cash or resources). It is a common misconception that physically smashing a hard drive up will render all of its data unrecoverable. In fact, at a recent tech expo in New York -- Wired magazine's NEXTFEST -- I encountered several members of Geek Squad, a tech support company whose employees are supposedly all computer experts, who were proliferating this very idea. They even had a display set up in which every few hours or so they would crush a hard drive with a hydraulic arm as a demo.
Shattering a hard drive doesn't work because, with a magnetic force microscope, data recovery experts can probe the surface of a hard drive platter and reconstruct the original pattern of 0s and 1s on the surface of the disk. One might suggest, then, that if the original pattern of 0s and 1s on the drive is modified using magnets, the data will be beyond recovery. Yes and no. Exposure to magnets tends to corrupt data on magnetic media, but not erase it; so, while this seems like a promising solution, it isn't very reliable. The NSA's standard for ensuring that a hard drive is thoroughly cleaned of any useful content is to rewrite the entire drivespace seven times with random 0s and 1s. These are low-level formats we're talking about, not a simple "format C:" procedure as carried out by fdisk; running a magnet over a hard drive won't cause nearly enough corruption to make it useless to the NSA.
So we're back to our original question: what to do? The best way to get rid of data on a hard drive is by altering the chemical structure of the platters -- put simply, torch 'em.
Melting your hard drive is an irreversible process that will leave the hard drive completely useless. There's no way to recover data from a small mound of soot and mangled metal. But if the Feds are already at your door, there's no easy way to do this -- you'd have to get some gasoline, perhaps, open the PC case, douse the insides, and light a match -- not to mention the fact that starting a fire under your desk isn't the smartest thing to do.
So, if you're the kind of person who's got extremely incriminating material on your hard drive and you're worried that the FBI might show up at your door one of these days, do yourself a favor and do a bit of preparation. One nice way to do this is to make use of a thermite reaction, an extremely unsafe but highly effective oxidation reaction that burns at 3000 degrees F. You might affix some volume of thermite to the hard drive, encase your entire PC tower in cinderblocks to contain the reaction, and use a model rocket launcher attached to a model rocket engine as a blasting cap to start the reaction. Since thermite was used originally to weld together pieces of railroad track, this kind of reaction won't have much trouble spreading to your floors, walls, and the rest of the building, so containing the reaction is of primary importance. If you've got this kind of setup, as soon as you hear The Man at the door, you press the ignition switch on the model rocket ignition pad, and within a few seconds, job's done.
Of course, it should go without saying that I don't take any responsibility for what you do after reading this post -- or ever, in fact -- and that everything I've written here is strictly for educational purposes. I'm don't advise that you engage in piracy, pyro-cy, defying the law, or anything else comparably foolish.