Suppose my small brother is playing magnets near my Dell laptop.
Will a magnet wipe data from the hard drive, or otherwise irreversibly damage the hard drive?
While you can damage hard disks with magnets, it takes a very powerful magnet to do so.
As this article explains, any magnet that your brother was playing with won't be large enough to do any damage:
This myth was popularized by movies where hackers or criminals would quickly erase the contents of their hard disk drives with a few sweeps of a powerful magnet. Unfortunately, it is almost impossible to do it with regular magnets, no matter how big they are.
Every hard disk drive actually contains two powerful neodymium-iron-boron magnets that control the movements of the read/write heads. Yet the data on the platters remain unaffected. It will take a very, very powerful magnet to affect the data inside the hard disk drive.
Science and a little bit of logic
I was curious about this, and as always on the internet, results are mixed.
There's a video here where a bunch of kids manage to do it with an aquarium magnet. I don't think its the magnetism that killed the hard drive, and its possible that its just a really old system. They tried smaller fridge magnets, and eventually a magnet the size of a blackboard duster used to clean aquariums. The system froze, and didn't detect the drive on the next boot at all .I'd note that the whole drive died as opposed to data loss, and that's not what one would expect from the myth, I'd expect weird failures but the drive being detected. Its probably a good example of this myth. I'd note though that typical smaller magnets were entirely harmless even if this were a valid example, and that the magnet is right on top of a running hard drive, and moved across, something hardly likely to happen by accident.
There's a seriously strong magnet pair inside your hard disk to actuate the 'voice coil'. However the layout is designed so that there's a VERY strong field between the parallel magnets and less outside. The presence of magnets inside a hard disk may not necessarily mean that its safe.
Someone's tried this at kj magnetics (link stolen from Michael Frank's comments, with his permission) and found that it didn't really wipe data. They tried a range of magnets, and as a magnet retailer, they probably have the good stuff.
I assumed that the casing of the hard drive would provide some degree of shielding, but the link reports that there might have been some deflection of the hard drive platters with stronger magnets. If you put the powerful enough magnet in the right place, you might cause a head crash, which is very very fun.
However the data itself seemed to be fine - and the explanation is that in creating smaller and smaller magnetic domains, they needed materials that were harder to 'flip' the magnetic state of by accident (so higher coercivity). As such they're also more resistant to magnets than older media.
Guttmann's paper on secure deletion of data has a nice table of these
Typical Media Coercivity Figures ______________________________________ Medium |Coercivity 5.25" 360K floppy disk |300 Oe 5.25" 1.2M floppy disk |675 Oe 3.5" 720K floppy disk |300 Oe 3.5" 1.44M floppy disk |700 Oe 3.5" 2.88M floppy disk |750 Oe 3.5" 21M floptical disk |750 Oe Older (1980's) hard disks |900-1400 Oe Newer (1990's) hard disks |1400-2200 Oe 1/2" magnetic tape |300 Oe 1/4" QIC tape |550 Oe 8 mm metallic particle tape |1500 Oe DAT metallic particle tape |1500 Oe
Which pretty clearly reflects why a old school floppy disk or tape was more susceptible to this kind of thing, and a modern HDD is not. Its plausible modern drives may have even higher coercivity.
That said, your laptop itself has a certain degree of magnetic shielding (your HDD caddy is sheet steel probably and there's a lot of metal panels and frames in a laptop. Chances are any magnet powerful enough to nuke your HDD would also be dangerous, and possibly cause your brother to loose a finger or two by accident when it gets attracted to something solid and steel, or worse another magnet. So... your brother should not be playing with magnets if they are strong enough to wipe your hard drive. However I'd be worried about your brother's fingers more than the hard drive.
There is however a way to destroy electrical components with a magnet. With a strong enough moving magnetic field, you might just might induce a current strong enough to burn out some component. Once again, this would need a VERY strong current, and very specific circumstances. As a kid, I once blew out a PSU by moving a pair of hard disk magnets along the power cord. Not recommended.
I leave it as an exercise to the reader to see if they can turn a door into a giant degausser or cause a head crash with a magnet on a HDD.
Chances are, its unlikely the regular household magnets you'd find commonly would do any harm.
You have two questions here.
Yes, magnets can damage your hard disk.
But, as Michael Frank pointed out in the comments they cannot wipe data.
... strong magnets certainly can damage a hard drive if brought close enough. Keep neodymium magnets away from good hard drives! (1)
It all depends on how strong the magnet was your brother was using. Most likely, it wasn't very strong and did no damage.
No, simple magnets can’t do the trick; they’re built to withstand that kind of stuff. However, don't try to hit it with a sledgehammer. My dad did once have some hard drive magnets lying around, and I accidentally got them stuck on the skin between my thumb and forefinger. They’re very powerful and unpleasant.
Even though they could do no real damage, though, I would still advise against allowing this to happen (you never know, right?).
There are some machines such as degaussers made to re-polarize the whole hard drive at once, generally used by the government after de-commissioning a hard drive.
The industry standard for a completely clean wipe is 7 passes for re-polarization. If data is overwritten, it is still possible by very high-ended software to recover the data, although I would expect it to come at quite a price. Past 7 times though is typically beyond the reach of anyone.