Hard disks and magnets

[WildW]

Most folks get paranoid about having magnets anywhere near hard disks, what with the data being stored magnetically. . . but I got to thinking, can you really affect stored data with a magnet?

Anyone who has taken apart a hard drive to steal the lovely strong magnets knows that there are really powerful magnets in there (surrounding the coil at the end of the actuator arm). The disk platters are constantly spinning in the magnetic field that these magnets create, and those magnets are stronger and closer to the platters than most of the other household magnetic hazards we could think of.

So the way I figure it, flipping a single bit on a hard drive must take a very powerful and tightly targetted bit of magnetism, or those strong magnets would screw with your data. At the same time, the "bits" on the disk surface are so close together, yet you only manage to flip the correct ones when writing data.

So. . . I reckon that you could probably wave magnets near a hard disk all day and do no harm. . . . though naturally I don't feel like trying it Am I horribly wrong?

 

[seemingly random]

The loss of data on at least one hard drive inside a pc sitting on the floor at work was attributed to the magnets on the front of vacuum sweeper. The magnets were added to pick up paper clips so they wouldn't get caught inside the sweeper roller. I never tried to verify this. I was safe since my office was cluttered and the sweeper couldn't get near the pc.

 

[Modelworks]

The magnets inside the HD are placed in such a way that their fields do not interfere with the platters. Take some of those magnets out of the drive and play with them. You will see that the major force is on the large flat areas, not the thin sides. The same thing goes for the spindle motors. The field is directed at the motor parts not the platters. If you were to take those magnets from the HD and place them on the sides of the drive and move them back and forth, you will corrupt data. It isn't just the action of placing a magnet on the drive, it is also the movement of the magnet back and forth.

A popular way for corporations to erase old drives is degaussing equipment. It places the drive inside a magnetic field that erase the contents. The one like below only uses magnets, no power required.
http://www.datadev.com/degausser-eom3.html

 

[TheStu]

Well, there is no doubt that a magnetic with a high enough Guass rating will wipe a hard drive, but the question for me is how much does it take?

Are we talking fridge magnet strength?
The magnets in my MacBook?
Magnets like what you might find on clothes?
Neodynium magnets?

Where does one draw the line and decide to no longer use their drives as a magnet holder?

 

[ShawnD1]

Originally posted by: TheStu
Well, there is no doubt that a magnetic with a high enough Guass rating will wipe a hard drive, but the question for me is how much does it take?

Probably a hell of a lot. A hard drive is magnetically shielded by the iron case specifically to prevent magnetic damage.

The magnetic shield is partly described by the shield's B-H curve. I can't remember exactly how this works, but I can try to describe it anyway. IIRC, the B field is what you normally think of as being a magnetic field and the H field is the magnetic field that can be contained inside a material (sort of). If you apply a B field (a fridge magnet) to the iron case, it induces an H field in the case. While the B field is lower than what the case can handle, the case's H field forms a closed loop around the hard drive internals and everything is just dandy. If you apply a very strong B field to the case, the case's H field becomes saturated and the excess B field is able to pass through the magnetic shield. Once the B field is through the case, it can induce an H field and voltage in the hard drive guts and destroy the data.
http://en.wikipedia.org/wiki/Magnetic_field#B_and_H

I've opened a hard drive before and the case was a few mm thick. I think you would at least need a very strong fridge magnet to get through that. The little crappy fridge magnets that fall off when you gently pull on them are not enough.

 

[Modelworks]

Originally posted by: ShawnD1

Originally posted by: TheStu
Well, there is no doubt that a magnetic with a high enough Guass rating will wipe a hard drive, but the question for me is how much does it take?

Probably a hell of a lot. A hard drive is magnetically shielded by the iron case specifically to prevent magnetic damage.

Hard drives have not contained iron in the case for many years. They are almost entirely aluminum cased now. Find an old hard drive and try to stick a magnet to it, it will not stick.

 

[Howard]

Originally posted by: Modelworks

Originally posted by: ShawnD1

Originally posted by: TheStu
Well, there is no doubt that a magnetic with a high enough Guass rating will wipe a hard drive, but the question for me is how much does it take?

Probably a hell of a lot. A hard drive is magnetically shielded by the iron case specifically to prevent magnetic damage.

Hard drives have not contained iron in the case for many years. They are almost entirely aluminum cased now. Find an old hard drive and try to stick a magnet to it, it will not stick.

On a side note, are the (natural-finish) aluminum plates constrained-layer damped? They do not ring, and when I view them from their thin sides, it seems as though the plate is laminated.