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Can magnets cause problems for external hard drives?


I recently bought an external hard drive, a Seagate Backup Plus. Is it vulnerable to magnets? I know that internal hard drives are vulnerable, but I’m not sure about the external ones. Also, if they are vulnerable, do all external hard drives become vulnerable as well or just certain models?

I look at how paranoid we (as normal computer users) need to be about magnets and hard drives.

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Magnets damaging hard drives

You know, to be honest, magnetics is something that I have never, ever worried about. There’s… I don’t know if I want to call it an “urban myth” or just a misconception that a magnet will do hard drive damage.

The best way to describe this idea is to trace it back to its roots. Back in the day, we didn’t use hard drives; we used floppy disks. Floppy disks were media that were enclosed in a kind-of plastic wrapper. You could get a magnet very, very close to the magnetic media so that even so a small magnet could scramble the data on the media… if you were intent on it, if you really ran it around a little bit.

It certainly wasn’t the case that data, even then, was easily erased. But it definitely was possible to erase it, even with a small magnet.

Magnetic erasers

In fact, there were bulk erasers, big magnets that you could get that would allow you to demagnetize or scramble the data on a stack of floppies all at once. It was using, like I said, a big external magnet, a big heavy thing.

Once we moved to hard drives, while technically, hard drives are magnetic media, the magnets that you and I typically have aren’t nearly powerful enough to have any noticeable impact on what goes on inside the hard drive itself. I don’t care whether the hard drive is internal to your PC or in an external enclosure; the magnets that you and I have simply don’t have the power to do any serious damage.

Now, there’s still a lot of debate on whether or not that big, heavy bulk-erasing magnet actually does anything to external hard drives. So, the fact that there’s a debate should right away tell you that it’s not the most effective way to erase hard drives.

And in fact, what it should also tell you is that, gosh, hard drives, if there’s even a question, aren’t really that sensitive to magnetism as perhaps we thought. External magnetism, that is. So, I certainly don’t recommend bulk erasers as a way to erase the data on your hard drive. I don’t believe it’s effective.

I don’t believe it’s a guaranteed erase by any stretch.

How big is your magnet?

And day to day, I mean, you’re not putting bulk erasers next to you external hard drive or next to your PC. You’re not putting magnets anywhere near that kind of power, near the devices you’re concerned about.

So practically, it’s just not something you and I need to worry about on a day-to-day basis.

So, the short answer is no. Don’t worry about it. Keep the big magnet that you might be using (like at the back of your super, large speakers, or the bulk eraser that you had from 30 years ago), keep that somewhere else – but for day-to-day stuff, for the average person, for practical use, no. It’s not even something to be worried about.

(Transcript lightly edited for readability.)

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12 comments on “Can magnets cause problems for external hard drives?”

  1. I was always concerned about putting a laptop in my car after I discovered how big of a magnetic field that car alternators have. 10 or 12 years ago, I had an old alternator in a crate and put the crate in my house about 5 feet from my TV (a tube TV, not the flat screens like today). Later that evening I noticed the color on the TV starting to look a little strange in the lower right corner but I wasn’t concerned at that time and didn’t even think about the alternator sitting 5 feet from the TV on the floor, directly away from the lower right corner of the TV (which was up on a stand). The next day the entire TV was completely discolored and the lower right corner was black. That’s when I thought about the alternator sitting there. I picked up the alternator, put it close to the TV and the entire screen shrunk to a pin-point. Needless to say I had to throw that TV out and when I got my first laptop several years later I remembered that experience and feared putting it in the car. I used to carry it in the trunk!

  2. Years ago I bought my son a science kit for his birthday that contained a strong magnet, (not the red horseshoe shaped one). Later I heard the other children laughing and went in to see him wiping it over the TV screen. The colours on the screen were affected as he moved it. Yes, it was an old tube TV. Not likely to try it with my LCD or plasma.
    My question would be not so much about the hard drive but the monitor. Could the same occur?

    Old CRT-type monitors and TVs can definitely be damaged by magnets being placed too close. They are very sensitive to magnetic fields, as magnetic fields are used internally to create the image that you see on the screen. This has no relationship to hard drives or any perceived hard drive sensitivity.


  3. The old CRT (Cathode Ray Tube) that was used in TVs before they went to various flat screen tecnologies were very sensitive to magnetic fields. The beam used to draw the image on the screen was aimed by internal electronic magnets and any magnet near the screen would mis aim the beam causing the many interesting effects.

    They actually left residual magnetic fields on the metal around the screen and had a demagnetizing (degaussing) coil around the outside edge of the screen that went on for a couple seconds whenever you turned the TV on.

    Too late for Gabe’s need but the same demagnetizing coil (I used to have a BIG one for eraseing reel to reel tapes) would have returned the tv to normal operation. Yes, I had fun playing with magnets and got to a point where the small degaussing coil in the TV was not strong enough.

    The flat screen technologies do not include any use of magnetic fields and are not affected. A dozen or so years ago I designed a tester for large (1200 amp) circuit breakers. We had to be able to produce 8000 amps in our machine which caused a very large magnetic field. Even at lower power levels the CRT monitor was completely unreadable because the way the image was waving across the screen. We bought an early plasma based monitor to mount on the machine since it would not be affected by the fields.
    We had a computer in the machine and even with that massive magnetic field, it had no effect on the hard drive.

  4. Regarding magnetic fields and CRT (cathode ray tube) type monitors: It takes a very small magnetic field to effect them. The process by which the display is generated in the first place is based on magnetic fields ‘pushing’ an electron beam around the screen. Most effects of that nature will not permanently damage the monitor or TV.

    Most monitors of that type have degauss coils wrapped around the screen that can be activated manually OR are activated every time the unit is turned on. The strength of field involved with those coils is MUCH greater than most other magnets you might inadvertently place near the screen.

    Maybe you have noticed how a motor operating near your CRT distorts the display? A pencil sharpener, a fan, a stapler? While that interference may be electrical in nature, chances are better it has to do with the magnetic field being generated by the accessory. Those fields don’t often damage anything, but they are irritating.

    Sill, ‘It’s not something to be worried about.’

    I read that somehwere. ;)

  5. About 10 years ago, my granddaughter put refrigerator magnets all over the metal tower case of my daughter’s computer. Nobody thought anything about it, but the computer slowly stopped working properly, until it just completely died. It was an old computer, but could this have been the cause?

    In my opinion it’s unlikely.

  6. Regarding magnets, I recently found a thumb drive firmly attached to a strong magnet. To my relief, it did not seem to effect any of the data on the drive.

  7. I’ve taken apart many dis-functional hard drives just to retrieve the strong magnets they contain. The magnets are VERY strong, and they are sitting less than a quarter inch from the platters. I think they control the pickup arm.

  8. I would add to the comments correctly mentioning the Degaussing Coils on CRT TV sets, that for those coils to function fully, the TV set has to be fully switched OFF to allow the Thermistor (Thermal Resistor) in series with the Coil, to cool down to ambient.

    At ambient/low temperatures, the resistance of the Thermistor is low, allowing a large current to pass through the coil; and it this surge of current and associated magnetic field to carry out the degaussing action.

    With some TVs, that surge current can be large enough that the interaction of its magnetic field with others around, can be heard as a “plop” or similar.

    The Thermistor heats up in about 30 seconds, with its resistance increasing, thus reducing the current and degaussing field to a minimum.

    About 1992, a colleague mentioned that his secondary TV set in a bedroom, was “purple”.

    It turned out that he never switched it off completely, leaving it in stand-by, thus the thermistor never cooled down etc.

    I told him to switch it ON and OFF about half-a-dozen times, with about 5 minutes to each OFF period.

    The next day, he reported that the worst of the “purple” effect had gone.

    I also carried out an interesting experiment when we first started using coloured CRT terminals into our main frame computer.

    I turned one upside down, so that colour separation due to the Earth’s magnetic field could be seen.

    This was then “corrected” by switching the terminal on and off several times.

    I then turned the terminal right-way-up; and again the interaction with the Earth’s field and colour separation could be seen.

    Once more going through the on/off sequence several times corrected this.

  9. depends on what is meant by “cause problems for external hard drives”

    A magnet that can flip or destroy bits on a platter without being in direct contact with the platters is not something that is available to the average consumer and would also be strong enough to pull the coins out of your pants pocket right through the fabric

    However, the strong magnets that are used to control the HDD Head actuator arm are used in such a configuration so that the magnetic field around it isn’t strong enough to change anything on the platters

    With that in mind though, if you take one of those magnets and slide it along the bottom or top of the HDD the motor that spins the platters will stall which could cause a head crash, especially if the HDD tries to move the heads when the platters aren’t spinning

    Even a reasonably strong fridge magnet can stall or slow the HDD motor

  10. I have both an External hard drive in storage(not in use, in a closet, until the next back up) and a bulk Video tape eraser, I do keep them really far away, when in use. I thought my Seagate had shielding. I will not take any chances, anyway.

  11. Long ago, maybe 15 years ago when floppy disks were enclosed in paper and people still played music cassettes, a politician running for office here in San Jose, Ca, as an advertisement, sent refrigerator magnets with his name on it to everyone in his district. I always wondered how many people received floppy disks and music cassettes in the mail that had been ruined by this dumb politician?


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