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View Full Version : Magnets, Hard Drives, and AlBooks


Toe
Nov 18, 2004, 03:26 PM
Getting a magnet near your hard drive will toast it, right?

Well, if you have an Aluminum PowerBook... take a paper clip and touch it to the latch-release button on the front. Move it around a little...

WTF?

That magnet is for pulling down the latch hook from the top. But what is it doing so close to the hard drive? Isn't that a problem? It seems like a pretty strong magnet....

jeremy.king
Nov 18, 2004, 03:44 PM
Give this a read.

http://seclists.org/lists/fulldisclosure/2004/Jul/0596.html

patrick0brien
Nov 18, 2004, 03:47 PM
Getting a magnet near your hard drive will toast it, right?

-Toe

Eee, err. Well, not exactly. Yes, an HD is a magnetic medium, but other than that quite different than a magnet. First off, a layman's magnet is essentially a block of ferrous particles that all point in the same parallel direction - that old discussion of north and south poles (dipoles). If you take that same block and point each dipole in a different direction, the magnetism effectively ceases because it is no longer focused.

In a hard drive the dipoles are highly sequestored. Think of a disc that has billions of little magnets stuck to the surface - each with their own set of dipoles.

In order to 'screw up' a particular magnet, you need to hit it with a magnetic field that intersects the direction the dipoles are pointing - preferably with a field at 90 degrees, though less an angle can still repoint the dipoles. Also let's not forget that each magnet has but a simple bit of information to carry - a bit. A 1, or 0: Yes or no: Magnetized or not. You'd need to really hit that block hard to change it's state enough to change that information. And you'd need to do it billions of times.

Additionally, to really screw up the dipoles of the billions of tiny magnets that make up the business layer of a hard disk platter, you'd need to hit each with a magnetic field equal to or greater than the field used to align them in the first place - the R/W head on the slider (very strong, microscopic, concentrated field), and the same basic size.

So! What does that mean? Well, to really kill an HD, you need to go out of your way, and degausse it. Meaning to get a field strong enough, yet wild enough to affect so small a magnets, you need to creat a strong flux, a powerful, vibrating, self-cancelling field, and get it really close.

So don't worry. BTW, that little magnet you found is on of two - the other, likely located along the right side of your screen, is to enable the computer to know when it is closed.