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?
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.)
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.
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.