[dpedu]

From 6 inches away? Unless this was a hockey-puck sized magnet I'm calling it a coincidence.

> (harvested from an old hard disk drive)

Okay, this is making even less sense. "Hard drive magnets", in a drive, are millimeters from the spinning platters. I refuse to believe that the same magnet, moved 6 inches away from the computer caused catastrophic damage.

[EvanAnderson]

I will definitely concede that the 6" number may be an incorrect memory. It certainly may have been closer, but it wasn't right on top of the unit.

If it was a coincidence it was certainly well-timed. I don't have the Thinkpad anymore, but I do still have some 2.5" PATA drives. It might be interesting to test this and make a video.

Here's a picture of the magnet in question (it holds stuff to my refrigerator now), w/ a penny for scale: http://mx02.wellbury.com/misc/20171203-Magnet_of_doom.jpg

This magnet was pulled from a Micropolis 9GB SCA-II 3.5" low-profile drive dating from roughly 1998 (I had a crap-ton of these drives and, as they died, I pulled their magnets, so I have a bunch of these). These particular magnets will stick to each other thru my 3" thick butcher block table. They are physically larger (substantially thicker) than the ones I've pulled from newer drives.

Edit:

I assume that the data and servo tracks written to the drive are done so in the presence of the magnetic flux of the magnets supporting the voice coil. I always just assumed that adding a substantial new source of magnetic flux (the magnet in my hand) either induced a current or magnetized some component in the drive.

[syshum]

>> They are physically larger (substantially thicker) than the ones I've pulled from newer drives.

Not really, they look very close in size to current Enterprise Drives (https://imgur.com/2HjuuKM), Of course I have removed the Metal backing plate from mine

Now consumer drives do have smaller magnets (that is one way to save cost)

Here is a photo of the 4 styles of magnets I still personally have, the smaller magnets are either out of consumer drives and 2.5in drives manufactured in the last 7 years https://imgur.com/59XjVPm

And just for fun a small assortment of my collection because why not

https://imgur.com/bW6ycRI

[EvanAnderson]

I've only had the opportunity to tear down a few consumer drives in the last few years. I haven't seen inside an enterprise-class drive in a few years. (I've stopped dealing with hardware directly for my Customers, and I haven't purchased much hardware for personal use over the last few years.)

I've found scavenged hard drive magnets to be very useful for odd jobs. One served several years holding up the fallen head-liner over the drivers seat in one of my crappier cars.

[quickben]

But it will mess up the power flow. Misbehaving powesupplies destroy HDDs.

[cptskippy]

So a magnet held 6 inches away from the PC did more damage than the one inside the HDD literally inside the PC?

[2muchcoffeeman]

Yeah, I hope people realise that everybody has these magnets IN their computer. You might have a NAS in which case you have several of these magnets much closer than 6 inches to the other drives and power supply and nothing happens.

Magnets near your or even on the computer do nothing.

[Scramblejams]

Magnets induce a current when moved relative to a conductor. The magnets you mention aren’t moving.

[zaarn]

When you swap drives, they are. Yet people don't have mysterious data loss when swapping drives out of their bays.

[sandworm101]

>> moved relative to a conductor

Yes. The magnet is stationary and the spinning drive plate is moving the data, the conductor, very rapidly relative to the magnet. (imho the real issue is the large metal box surrounding the drive plates. It's hard to feel a magnet from inside a metal house.)

[harshreality]

So what you're saying is that nobody can hot swap drives in a NAS without destroying neighboring drives? I don't buy that either.

[quickben]

No, what he is saying, is that moving magnetic field, induces current, and can mess up the cheap power supply and/or other components, which can then mess up the drives.

In a server, the power supply is on the other end of the chasis. In a laptop, it's a lot closer to the internals.

> I don't buy that either.

Theory: https://en.wikipedia.org/wiki/Electromagnetic_induction

The forward model: https://www.youtube.com/watch?v=UL_ryxub-RA

The reverse model: https://www.youtube.com/watch?v=J9b0J29OzAU

The big ones: https://en.wikipedia.org/wiki/List_of_conventional_hydroelec... Note: the Three Gorges: 22 GW

For comparison, nuclear power: https://www.nei.org/Knowledge-Center/Nuclear-Statistics/US-N... Largest U.S. nuclear plant: Palo Verde (Arizona) produces around 4 GW.

For home applications: http://www.ebay.com/bhp/hydro-electric

[ars]

I agree that it makes no sense. A magnet outside the case is not going to do anything to the platters.

kees99 posted a possible explanation, where the magnet moves the voice coil, which ends up destroying the drive by writing data in the wrong places.

[dpedu]

This can't happen either. Early hard drives (and I mean early: the 80s) needed precise head alignment (in respect to the how far the head is from the center of the platter) in order to read or write data in the correct location. If you've heard of "low-level" formatting, that's when a on-platter layout unique to the head and head movement mechanisms of the host drive is created.

Modern drives, however, are designed to "follow" the tracks that are cookie-cutter imprinted onto the platters. We use this design now as it's far easier to design a head that reads tracks and makes slight adjustments to stay on track, vs a head that needs to move to an absolute exact position.

In other words, modern drives are designed to combat this exact problem.

[Waterluvian]

I stack hard drives and they keep working.

[EvanAnderson]

Absolutely. Hard disk drives don't stick to your refrigerator (like a naked magnet does) either. My layman's guess would be that the fields generated by the magnets in opposition in the voice coil are somehow "balanced", so that the net magnetic field produced by the drive is negligible.

[SAI_Peregrinus]

It's simpler than that. They're held in an iron ring. This confines the field nicely, the permeability of the iron is quite high.