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We’ve discussed what it means to actually store information on a hard disk drive, how you magnetize it and how you pull that information off. Neat stuff, but a bit heavy on the abstract physics. Today we’re going to zoom out a bit and look at the mechanical bits of how hard drives work. Here, let me start you off with a picture. Take a look at these two hard drives (conveniently cracked open for viewing purposes), one I borrowed from the boss man, and the other I picked up off the “Free Stuff” shelf when they moved the engineering department. Tell me which you think stores more data:
Now I’ve actually got no idea on the history of these two drives, what year they were built, their listed capacity. Couple points of difference tell me the one on the left is older, and stores far less information:
- The platters have a larger diameter
- The brown-orange tint on the disk
- That yellow thing on the side
Right away that color is a dead giveaway; it gets that tint from the recording medium. (And, if I’m being fair, from the reflection of wooden kitchen cabinets. There is a difference in the color, but it’s a bit hard to get on film.) On the older disks, that recording medium was iron oxide. Yeah, rust. Somehow marketing departments weren’t too eager to talk about it that way. “The Corrosion 5,000, with high capacity Rust-based storage!” The copy practically writes itself. The industry moved away from rust because you get better areal density (information per square inch) with different materials, even before that whole magnetoresistance thing.
The yellow thing on the side, by the way, is a ramp. When the hard drive powers down the arms swing over to the side and rest on the ramp. On older hard drives you just rested the thing on the platter. The ramp helps prevent damage from dropping your hard drive, and keeps it from dragging when you’re just spinning your disk up to speed.
Okay, but why the shrunken platters? Don’t we want more information on there? Because we can get so much areal density in other ways; using that extra space becomes less important. “I can store a thousand times more information per square inch, but you only get to use a quarter of the space.” Do you take that deal? Of course; you’re still working with two-hundred and fifty times your previous storage space. But why not go for the full thousand? Larger platters have some drawbacks.
- Speed: Let’s say you’re reading data off your hard drive, and let’s say it’s sorted like my sock drawer (not at all). You grab some from the inner ring, then you go all the way to the outer ring and grab some more, then you’ve got to get back to the inner ring, and so forth. If you have a smaller disk, you’re getting things quicker because you have to wave the arm over less distance. That’s true even if you have a well-ordered sock drawer. You can (if you’re a maniac) take that further. My brother @SamRhody once considered reinstalling all his games to be closer to the hard drive’s spindle to improve speed when gaming. If you’re that worried about it, get a solid state drive (which, to be fair, he did). [Note: Chris B. was kind enough to correct this in comment #6.]
- Flatness: that disk, it isn’t an ideal surface spinning around. There’s always some difference in height. That difference can be really important if you’re expecting your needle to float three nanometers off the surface. It’s easier to make a flat disk when it’s smaller. You have to keep it flat over less area. Smaller disks and shorter arms are also less liable to break from dropping.
- Momentum: It takes a certain amount of energy to spin one of these disks up to the 5400 or 7200 or 10,000 RPM you’re looking for. Pretty obviously that energy is going to be less if you’ve got a smaller platter. Less mass to swing about. And that translates to less energy to keep it spinning, less noise from the motor, and overall a cheaper drive to operate.
On the subject of those motors, let’s talk about ’em. There are two in your drive, one that moves the arm around and one that spins the disk. The one that moves the arm used to be a stepper (a stepper motor is one that allows you to specify exactly how far you want it to spin by changing which coils are active. Lets you do precise controls). When the precision required for positioning got too much for stepper motors the industry moved to voice coils. A voice coil is a fine coil of wire that moves something magnetically. You’ll find them on speakers, and they’re the things that compensate for your shaky hands in your cellphone photos.
(Actually, my company has been building a better solution to that cellphone camera problem. We use something called “shape metal alloy”; a wire that changes shape when you run a current through it. Just putting in a quick plug, and please direct all pricing inquiries to the sales team.)
The voice coil is the copper oblong ring on the right. If you look back up at the first picture you’ve got a big metal chunk sitting above the voice coil and obscuring it. That’s a magnet. You run current through the copper coils and you generate a magnetic field. That’ll push off of the permanent magnet above it and move your lever. Lever? The circle-thing (in technical terms) right next to it is a pivot; the actual read/write head is way over on the left. You’ll note you get more distance moved out of it than you put into the voice coil. That black thing on the left is a plastic support for the display model, it doesn’t actually show up in a hard drive. Here, lemme give you another photo. This one’s of that older hard drive from the first picture:
There’s still a black thing, but this one is much smaller. It’s there for its aerodynamic properties. Helps the thing float above the platter on the air currents generated by the spinning disk. You’ll also note that threat are multiple arms for multiple disks. Sort of obvious, in retrospect. Each surface of the disk can contain information (sometimes they leave one blank for marketing purposes). The discussion about why you make the disks smaller also applies to why you don’t just keep stacking disks on there. With one more wrinkle; it’s also harder to keep the various arms in sync the more of ’em you have.
The spindle motor is a servo that spins the disk. It’s got a couple of problems to solve. It’s got to run at high speeds (up to 15,000 RPM in some cases, 7200 RPM is most common), it’s got to maintain that speed very closely, and it’s got to be able to spin up and down to conserve power in between cat videos. (That is, I assume all you degenerates carefully copy all your cat videos to local storage in case YouTube goes down. Some things need to be secured.) The stator is fixed to the disk case, the rotor is fixed to the platters, and in between ’em there’s a ‘fluid bearing’ (oil) to keep the friction down. You can read an interesting bit here about a company extracting data from a hard drive whose spindle has seized up. Sample Quote:
Speaking of lubrication, our data recovery engineers have seen on at least one occasion a previously-opened hard drive whose owner had sprayed WD-40 onto the motor and platters to try and get it spinning up again. It was not a pretty sight.
That about covers it from the mechanical side. I’m figuring on two more posts discussing hard disk drives before we move on to other topics. Join us fortnight next when we discuss recovering data from dead hard disk drives, and how to make sure no one can recover yours, in “Delete my Browser History” or “What, With a Cloth?”
This is part twenty-six of my ongoing series on building a computer, the rough-and-tumble cowpoke way. You may find previous parts under the tag How to Build a Computer. This week’s post has been brought to you by the Singing Cowboy himself, Mr. Gene Autry! Gene Autry comic books are now available in the discount bin at your local comics store. Pick yours up today!