Is it possible to erase a hard drive with magnets?

a hard drive covered in magnets

The little, curved, shiny things are “hard drive magnets”, a metal bracket with Neodynium magnets attached. They are the strong, rare-earth permanent magnets that form half of the motor that moves the read-write head quickly across the surface of a hard disk platter. They are so powerful in fact, if you stick the flat ones to your fridge you’ll need to pry them off with a screwdriver. Most computer enthusiasts look at a picture like this and conclude right away that the data on that drive is toast.

There’s this old myth you can mess up or even erase the data on your hard drive by placing a whole bunch of fridge magnets on the outside of your case. To test this myth, I thought I might go one step further and cover a hard drive with really strong magnets to see what would happen. It seems sort of fitting that the strongest magnets I have are also the very magnets that make a hard drive work. I left them on for a day and a half.

The result? The data on the drive above was accessible after I pulled all the magnets off.

Since the drive pictured above was a little dodgy to begin with, I tried the procedure a second time with a different, known-good hard drive. As with the first one, there was no damage to the data at all. Not only did the second drive pass a CHKDSK, MD5SUMs of three ISO images on it were the same both before and after a sixteen hour period of being covered.

It would seem that hard drives are not as susceptible to magnetic fields as people think they are. This makes sense when you think about it a bit. Those powerful hard drive magnets are normally found inside a hard drive less than 2cm from the platters. Hard drives wouldn’t work at all if these magnets could erase the surface just by being close by.

The scientific reason why my ghetto drive erasing system doesn’t work is because for all their apparent magnetic power, the rare-earth hard drive magnets are simply not powerful enough to affect the particles on the hard drive platter when sitting on top of its cover. The coercivity of the magnetic material on a hard drive platter is very high, around two thousand Oersteds, and even though rare-earth magnets can have five times that level of coercivity, mine aren’t nearly so strong. Since there is a bit of space between the drive’s top cover and the surface of the platter, the affect of their magnetic field is even less. I could have a hundred of them piled on for a week and it wouldn’t make any difference.

So is it possible to mess up the data on a hard drive with permanent magnets? Sure, but not with the little ones you tend to find just lying around. Certainly not with fridge magnets. A big rare-earth one would probably do the trick, just as a bulk electromagnetic media degausser would. If you really, permanently want to wipe your hard drive, you could always check out the GuardDog prototype.  It not only erases a drive using a strong magnet, it destroys it.

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16 Responses to Is it possible to erase a hard drive with magnets?

  1. Jordan Burkhart says:

    Would it make any difference if the hardrive was hooked up and and running?

  2. cobolhacker says:

    Probably not. It’s the stubbornness of the particles that make up the media that keeps them at their set polarity, not whether they are moving or not. If the magnet isn’t strong enough, then it isn’t strong enough. It’s true that electromagnetic degaussers work by alternating their field back and forth, but I think this is to ensure a more thorough job. Unless your magnet is really strong not every particle will flip on the first pass.

    I’m sure someone is going to wonder why it is so easy to erase a floppy diskette. Diskettes are easy to erase because the coercivity of their media is only around 700 Oersteds or so. A strong fridge magnet will pass that and can be placed practically against the media.

  3. CyberFoxx says:

    Hmm, guess (E)IDE drives these days are alot better than the MFM/RLL drives of old. I remember messing up a number of them just by placing them near a good sized Hi-Fi speaker. If I remember correctly, there was no permanent damage, but a Low-Level Format was needed in order to use the drive again. A standard High-Level Format wasn’t enough and would most often fail.

  4. cobolhacker says:

    The surface materials have improved over the years. Twenty years ago hard drive surfaces were probably no better than floppies.

    Actually the magnet on the driver in a large, high-quality speaker is probably enough to screw up the data on a modern drive at 1cm or less, perhaps more. They are also made out of Neodynium and can be quite strong.

  5. Alex says:

    Another reason why it (a magnet) probally won’t mess up your hard drive is that it is almost always coverd by and aluminium housing that hosts the disks. This is also why when you take a hard drive apart there is a diffrent section dedicated to the houseing of these NIB magnets that help write to your hard drive. As in the picture you show you can see the outer cover that the magnets are sticking to. The magnets you see that are sticking are attaching to the steel(or iron basded) cover that usually goes over the outer electionic components that tell the hard drive how to function including the controll of the drive arm. Although this would reneder a drive unfunctional this would not erase the data off the disks. But most would consider this drive trash if the control data is erased by the magnets.

  6. Alex says:

    Also most of the magnets you show in your picture are sticking to themselfs. If you want to get a ture NIB(neodymium iron boron) magnet you need to use a flathead screw driver and seprate the magnet from the nonmagnetic surface ther are glued to. For examle that magnet that you have the screws atracted to is not a true magnet because of the backing you still have attached to it. thus rendering the north and south poles of that magnet limited. If you seprated that thin 1/16th IN to 1/8IN or so magnet from the back you could achive greater allover magnitisim. Same with most of the magnets you have in your pictutre thus rendering the fill potential of thoes magnets lesser.

  7. cobolhacker says:

    It’s true, all those magnets have only the one pole to work with. But they can still suck a screw off a table at 4cm. That’s still a magnetic field. Having an aluminum cover in the way doesn’t make any difference, because aluminum is not a good magnetic shielding material. The magnetic field goes right on through, just as is does through plastic, wood and flesh. If you take a 2mm plate of aluminum, put one of these magnets behind it, it will still suck screws off the table at 4cm.

    Your comments about the drive itself puzzles me and I feel I must set the record straight because it sounds like you’ve never actually dismantled a modern hard drive before. There’s no iron in a drive’s chassis. Most of the magnets you see in the picture are just sitting there. Some are attached to the steel bracket on the side I didn’t take off, but that’s it. There’s no iron cover over the electronics either and I’m not sure why you think there would be. I’ve never seen a desktop hard drive with iron shielding over the electronics. Seagate used to put an aluminum cover over, but that was to protect the electronics from getting bumped. In fact, there are no electronics actually inside a modern hard drive. There also is no ‘different section’ for these magnets in a drive.

    Hard drives look like this inside. The drive arm magnets are partially visible in the right of the photo.

  8. jason says:

    hit it with a magnet while its spinning – that’ll kill it

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  10. DrLex says:

    I can tell you it will most likely make a difference if the magnets are applied while the disk is running, because I did that experiment myself with an old drive. It had an aluminum cover, so the internals were as good as unshielded from external magnetic fields.

    First I tried the same experiment as reported here, by moving over the stationary drive with the strongest and biggest neodymium magnets I could find. No effect at all, the drive ran fine after that. But when bringing a magnet near it while it was spinning, it could be heard slowing down. At a certain point it started making all kinds of clicking noises, and that was the end of it. Even with the magnet removed, it would attempt to spin up but failed consistently. It was dead.

    Without any doubt, the data on the platters was still unaffected, but the magnet seemed to have damaged the drive’s motor or something else mechanical. Either it messed up the motor, or caused a braking effect through eddy currents in the spinning platters, causing the heads to crash. Anyway, the morale of this story is that it’s still a good idea to keep magnets away from hard drives. And it’s not a good idea to think a disk can be securely erased with a simple magnet. If the FBI wanted to know what was on my experimental drive, they would simply have to fit it with a new motor and heads.

  11. cobolhacker says:

    Might have messed up the motor for the arm, too. That could cause all kinds of havoc inside the drive, head crashes, failed seeks, etc.. The magnets I was using for the experiment were the very magnets that move that arm so it is not unreasonable to assume that they could affect it while working.

    I never tried this when the drive was actually working, so this is a cool finding.

    You are right about data security. You get yourself a copy of DBAN and have it write garbage over and over to the platter of the drive. If you really, really want to be secure, you then destroy the drive.

  12. mark says:

    i friend of mine told me if someone puts a round magnet over my security alarm pad ,that they can turn off my alarm .is this true

  13. cobolhacker says:

    Over the keypad? I really doubt it. Easy to simulate, tho. Open up your door and hold a rare earth magnet in front of the pad. If it does indeed work, you should talk to your alarm company. Either that, or get a crowbar and go on a crime spree.

  14. scsi 80 pin says:

    First of all it is very easy to simulate,and its too much secure and usefull.thank you

  15. We learned something new information.Thanks a lot!

  16. Really Secure says:

    What would the effect be if you took the platter(s) out of the hard drive casing, and rubbed a Neodynium magnet over the platter? Would that erase the data? I’m thinking in the case if the hard drive is defective (doesn’t work) but would like better assurance that the data would not be recoverable.

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