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I know roughly how hard-drives work; that the hard-drive involves multiple platters and multiple heads. A platter is a disk coated with some magnetic material and capable of being spun at high RPM. The drive-head has the capability to change the domain orientation on each magnetic dot to register as a 0/1.

What has me flummoxed is that adjacent drives are kept so close together without deleterious effect. Is the field strength so weak that it has no impact on it's adjacent platter? If not, how is the interference of adjacent platters cancelled?

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I don't know that everyone will agree, but I think this is a good on-topic question. It fails the "Is this a real problem that you face?" test but I think it's interesting enough from a learning-about-technology standpoint that it should stand. –  Shinrai Sep 12 '11 at 19:45

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It's just that weak, you're exactly right. The read/write head is VERY sensitively calibrated, and even it can't pick anything up unless it's perfectly in alignment. We're talking about regions measured in nanometers or smaller here - even if they were strong enough, they'd cancel out in aggregate at the distances in question anyway. (In fact, they're so small in modern hard drives that they're sensitive to THERMAL perturbations, which is why most drives these days have redundant magnetic layers on a single platter.)

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Actually it is still an issue on these new ultra high density platters, they solve it with firmware revisions during testing and development. –  Moab Sep 12 '11 at 20:05
    
@Moab - True, I'm oversimplifying. It's pretty interesting stuff if you get into the engineering of it. But, basically the upshot is "No, there is no issue with the end product" which is what I was trying to convey. –  Shinrai Sep 12 '11 at 20:09

Don't know it the drives really have all those platters and heads. Most I've opened and seen were just 2 platters, 4 heads, and were very distant apart. The newer, bigger ones, have 4 platters.

ex.: http://www.maximumpc.com/article/news/western_digital_adds_new_750gb_and_1tb_35_hard_drives_enterprise_lineup

The interference would be bigger in the same platter: think about a lot of magnets in a row, with the poles pointing up or down: this distance, to put 250GB = 1000 G bits in a single platter, would cause much more interference than one platter to another.

And the weak, it doesn't interfere in the near signal.

Also, take a look at this site to learn a bit more about magnetic density.

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+1 : Fun article on tested.com I didn't even know the magnets are oriented standing up!! –  Everyone Sep 12 '11 at 20:11

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