dknguyen

I've always known the concept that harddrives work at, but I've never really thought about it until now when I'm getting an external HD to back my stuff up since my mountain of DVDs is becoming too unwieldly.

How on earth does a harddrive manage to maintain that nm spacing between head and platter (like I'm pretty sure computer desks are not that stable, let alone I-Pods or notebooks)? Let's not even talk about how the reader manages to maintain it's lateral position over a ridicuously small area on the platter that represents a bit with all the movement going on around (even the vibration of the motor or external ones like the desk shaking).

And how does the arm move from center to edge at 60 times per second without flexing (I really want to see a video of that if anyone knows of one)?

Is it that flexing on the order of nm is much bigger than I imagine? Or that harddrives actually don't get read often enough to make a difference (RAM being accessed much more often than the HD once the data has been loaded on). Even if those were true though, you would think that bad data would be written a lot more often than it seems to be since I often jerk my entire desk when I spin my chair around and the arm rest hits the edge. Or has anyone ever actually tried jerking their harddrive while it's in the middle of writing a big file? You would also think that even when it's powered off and you are moving the tower, I-Pod, or notebook around that when you place it on the table it would cause the head to smash into the platter.

3v0

Unless things have changed since I last checked...

The head fly above the platter on a cushion of air. This keeps it from touching the disk. When the disk slows down the head is moved to a parking zone which contains no data and is allowed to land. When off the head is in contact with the disk.

If you open a disk drive you will see they addressed the arm flexing problem by using a triangle shaped arm. Tough to deflect/flex a triangle.

Take an old disk drive that boots and open it up. If you do it in a fairly clean area you can get it working without the cover. Watch it work and die.

dknguyen

THat cushion of air thing is something I don't understand. EVen though the HDs use a cushion of air, they are not sealed from the outside atmosphere right (except for models built to operate at high altitudes), but instead of have really fine air filter to the outside (I'm not sure how they made a filter that can filter out every speck of dust). THat's one of the reasons they need to be within a reasonable pressure to work. What I don't get is how does the air act as a cushion unless something is moving at really high speed or is pressurized.

kchriste

The platter spins at 7200rpm so it is moving relatively fast in relation to the read/write heads and a thin layer of gas/air is moving with the platters. Kinda works like the ground effect on an airplane but plane is still and the air/ground is moving instead.

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Sceadwian

Common hard drives work at 5200 RPM, most modern drive work at 7200.
If the inside of the disk is 1 inch away from center then that's about 5 miles an hour relative velocity with a 5200 drive, almost 7 on a 7200 drive. It just gets faster as you move to the outside. All you need is a carefully designed airfoil and a neutral spring and it'll keep it's distance just fine. If you listen to your hard drive when it first spins up by itself you can hear it coming up to speed stabilizing and the head lifting into an access position. Once moving ground effect will keep the head from hitting the disk even during pretty decent shocks. Laptop HD's have a freefall detector which immediatly parks the head if it senses an impending high G shock.

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hjames

The head is on a spring steel suspension - lots of flex in the vertical direction, not much lateral. (Note: You've never popped open a drive before?)

Also, where the head touches down, they actually texture the surface of the disk to reduce the stiction effects and make the spinup easier.

As for the tracks, they are initially written out on a very precision machine, or may use some sort of fixture to externally move the head and write out the low level formatting info. Once they are written, the drive just rewrites these tracks, keeping the same position - these tracks are the only way the drive knows what the current lateral (and radial) position is.

If the drive gets bumped hard enough, and the head gets knocked off the current track, and if you happen to be unlucky enough, I'd imagine that the drive would mangle the data, possibly bad enough cause that part of the drive to get remapped to a spare sector. When reading, the drive will know that it got jostled off the track and will probably automatically reread it, as
long as the data didn't get physically scratched off.

Recently there was an auction where they were selling off equipment from Maxtor (it got acquired by Seagate), and there was a lot of drive related equipment being sold off. Interesting stuff.

http://www.hitachigst.com/tech/techlib.nsf/techdocs/9076679E3EE4003E86256FAB005825FB/$file/LoadUnload_white_paper_FINAL.pdf

dknguyen

I've never bothered popping open a harddrive because I've never had one that died on me (I also have only ever owned one computer with my own money- all the ones before were bought by my parents so I couldn't just go popping up things).

3v0

There are loads of good disk drives around that are too small to be useful. The often have Win98 or XP loaded on them.

Just a suggestion:
It may be fun and education for you to get you hands on a old working Win98 or DOS box and tear it apart and put it back together. Many of the Win98 machines actualy do a creditable job of running MPLAB.

A comment:
There are a lot of things like the disk drives where the level of technology has advanced step by step over time. If you look at the most advanced state of the technology it is somewhat fantastic. If you walk through the steps that lead up to the current state it makes a lot more sense.

The first hard disk I had contact with was hooked to an IBM 1620. It leaked hydraulic fluid.

hjames

Ask anyone who does computer related stuff for companies, and they'll probably have some failed or ancient drives floating around. Computer fests also might have people getting rid of broken drives for $5-$10 or so. Hanging around universities is also neat - I remember messing with a discarded drive that had a stack of 14" platters. That thing had some serious inertia.

Andy1845c

I have an old one that you can have if you wanna pay shipping lol Still works as far as I know, its got windows ME or something on it.

I just junked out a few for the magnets. Old computers are all over the place these days. Are there any appliance recyclers in your area? They would probibly give you a few.

j.p.bill

Hey 3v0,

Was that a 1311 drive? I suffered with one of those for about a year. I also learned about IBM and bad hardware designs from that experience.

3v0

Gosh I can not say for sure but it sounds about right. Too long ago.

3v0

Dknguyen,

If you start asking around I am sure you can find an old computer or two that people would be happy to have you haul off.

At times you can find good working ones in the trash.

Andy1845c

I have got several that way. Last fall I picked up a perfectly good 300MHz pentium Gateway off the curb during our neighborhood cleanup. My first computer was a 300MHz Pentium and I paid around $800 bucks for it in 1998 Sad isn't it....

dknguyen

I actually have an old 500MHz HP that died on me...well it didn't die so much as it won't retain a Windows installation without permanently booting up in DOS within 4 restarts. I never found out what was wrong with it but I've never bothered to hack it apart because I don't know what's working and what doesn't! Needless to say the HD is still working fine because a new HD did not fix the problem.

I guess as far as computers, VCRs, CD/DVD players, and other things you find commonly around, I just don't really care how they work. Electronic things that are connected to some motors that visibly move something tend to spark my interest more.