Electromagnetism and Hard Drive Destruction
I received some great feedback about my article about boost converters in the “*last edition of Tech Tails.*”:http://blog.smalldog.com/techtails/tt917/ That series is certainly one I plan on continuing and am also forming a list of other things that I will be writing about. This week, though, I wanted to write about hard drives.
Platter-based magnetic hard drives all eventually wear out because they have moving parts. The bearings in the motor can dry out, the read arm can become misaligned…it’s a miracle that with the precise tolerances required hard drives work at all. When they do fail, it’s important to destroy them properly so that data cannot be recovered off them.
There are a number of ways to destroy hard drives so that the data is not readable on the platters. If you have a good drill press, you can drill right through the case and platters. There are also degaussing machines that attempt to remove (to nearly zero) the magnetically stored bits on the platters. The Department of the Navy lists bending, drilling, cutting, shredding or melting as acceptable platter destruction techniques.
Recently here at Small Dog HQ, we began cleaning out boxes of old PATA hard drives. The data on these drives, while old, still might have value, so it was important that we destroyed them properly. Since we don’t have a good drill press here, I took to disassembling the drives and removing the platters by hand. With the platters removed the empty shells can be recycled as e-waste.
Eventually, I had quite the stack of silver platters sitting on my desk. Originally, I planned to simply drill holes through the bare platters, but this proved to be time consuming and hard on the portable drill and bit we were using. Instead, I opted for a hammer and nail setter to put dents all over the platters before bending them. I set up the first platter on a block of wood, knocked it a few times with the setter and done. Easy! I grabbed the next platter, aligned the setter, swung the hammer and SMASH! The platter fractured into hundreds of tiny shards!
This is an important lesson I’m surprised I didn’t already know; hard drive platters can be made of aluminum or glass. The aluminum ones dent easily and will not shatter, but the glass ones definitely will. Now I had a problem. How on earth was I supposed to tell which platters were aluminum and which were glass? They look identical, they feel identical, they even weigh the same.
There are a number of ways to distinguish aluminum and glass platters, but this one is my favorite and was 100% accurate when I used it. It involves using one of the strong neodymium magnets inside the hard drive case. If you take the magnet and place it flush against the platter and drag it across, you will feel some resistance if the platter is aluminum. Almost like it’s sticking to the platter. A glass platter will exhibit no resistance. Of course this is not possible because neither glass nor aluminum is magnetic. What’s going on here? Aluminum is not magnetic, but it can carry electrical current. Physics 101: Move a magnet past conductive metal and you’ll create electrical current. In the case of the platter, the currents being generated will have magnetic fields that exactly oppose the original change in magnetic flux (the one created by dragging the magnet across the platter). This result is predicted by something called “Lenz’s Law.”:http://en.wikipedia.org/wiki/Lenz_law
The induced currents (called eddy currents) and their respective magnetic fields act to interfere with the motion of the magnet being dragged on the platter. This is why you can feel resistance as though the magnet is sticking to the platter. The same effect can be demonstrated more dramatically with neodymium magnet and a straight copper pipe. Copper is also non-magnetic, but if you drop the magnet through the pipe, it will fall far more slowly than if it was falling based on gravity alone. Once again, eddy currents and magnetic fields are being created by the moving magnet and acting to cancel out the original change in magnetic flux. In this case, gravity wins out, but not before the magnetic fields slow the magnet’s descent substantially.
So there you have it. A neat application of a very basic and important law of electromagnetism.
