While I'm about to look at hard disk drives (HDDs), the following is pertinent to any form of rotating storage media. As a little background first, I had to write off my laptop about a month ago after a hardware failure. It had been my primary machine for several years, or more correctly my only machine. Mid-July, I replaced my existing wearable system with a modified off-the-shelf handheld. I have no regrets whatsoever about the device, and it's largely taken over as my mobile computing solution. Heck, I'm using it now, strapped to my wrist, sitting on a rooftop in the rain. I'm replacing the laptop with a desktop system. As one might expect from me though, it's no ordinary desktop. The design specifications basically include just about anything up to and including nuclear war. And yes, spend as little money as possible on the project. You might one day see the project on Hack a Day.

The ultra mobile system, among other advantages uses entirely solid state storage. As recognised by the OLPC developers, hard drives are one of the primary points of failure. While solid state storage might be faster, more robust, smaller and more energy efficient, it's currently far more expensive than equivalent amounts of magnetic media. Even if it weren't so expensive, I was trying not to have to buy components if I could help it. Thus I am in the position of booting from flash but keeping the bulk of my data on hard disk.

Remember this machine has to be able to take significant abuse, and well, leaving the hard drive permanently parked would kind of defeat the point of having it. "Laptop" hard drives, 2.5" ones, are generally more resilient to shock and vibration than 3.5" ones. Yet another reason why the choice was obvious. Some mobile computer systems also include a number of accelerometers which allow for two things. Firstly, by preempting or detecting dangerous levels of vibration or shock, the hard drive controller is able to spin down platters and park heads to minimise potential mechanical damage. Secondly, knowing the geospatial orientation of the platters allows for an enhanced level of control over keeping heads the correct, minute distance from platters. Fixed installations in general aren't really expected to move, and therefore obviously lack these sensors.

Rotating platters act as gyroscopes. This is significant because it means that they will attempt to keep themselves aligned perpendicular to the horizontal or vertical plane, whichever is closer for them. Now, while platters might self-align, the heads that read and write them have no such "intelligence," the result being fragile platters forcing themselves into equally fragile heads. The heads may be bent, and/or platters scratched. Mounting the drive in a gimbal detracts somewhat from portability, sorry. I take it you realise already that perfection is a dream?

Back to movement in a plane. A380? ...That's not what I meant. Being bounced up and down is mechanically possibly a drive's worst nightmare. In case I didn't make it clear enough, those heads must not touch the platters they hover a fraction of a millimetre above. Did someone say above? Yes, like a gramophone, the head is mounted above the platter. Contrast this with optical storage, CDROMs for instance, where data is stored on the bottom of the platter, and the head is (logically) underneath. This makes perfect sense in a top-loading situation, but why it's rarely found in magnetic media I've no idea. Perhaps because the innate weight of head arms tends to bend them downwards. Modern hard drives don't really seem to have a "right way up," and will happily operate with the controller board either above or below the platter assembly. Depending on whether kinetic energy is more likely to be travelling up or down in the drive during mechanical shock, one may choose to have the head above or below the platter in certain circumstances. Remember that that which goes up must come down, so YMMV.

Cushioning material is frequently used in mounting hard drives to dampen vibrations not only produced by them (both in and out of the audible spectrum), but of course also external ones they are subjected to. It might sound cool at first, but magnetically levitating a hard drive to hold it in place really isn't such a good idea. Rubber bands will dry out and crack, but are found manufacturer-supplied in a number of enclosures. How about placing them inside a tank of water? That worked in the Dambusters. Uh, hard drives don't float, that won't work. Ocky straps / bungee cord? Could work, might take up a bit of space though. In fact, this does work rather well for installations with significantly more space. Foam? No, not like firemen spray at blazing jet fuel, like car washing sponges are made of? Yes, excellent. Provided it's supportive enough, foam is very good for shock-absorption. Just please, don't make a sandwich with your hard drive in the middle of two sheets of foam, this is really bad for heat dissipation. How 'bout springs? If done incorrectly, conductive springs could cause a short circuit on the controller board, even non-conductive springs could damage the board, and anything pressing down on the lid is a bad idea as it will cause the platter to bind. Otherwise, yes, go for it. Rubber? Yes, a softish rubber is excellent for this purpose. I have little bad to say about that. Just, um, ordinary screws? Er, we're trying to minimise mechanical shock here, guys. Boobs? ...I'm going to pretend I didn't hear that.

So, what do the pros use? Who exactly are you meaning by "pros" there? As an example of a commercial setup, Apple have something pretty sweet going in their recent portables. I might not much like the company most of the time, but they stand out from the crowd somewhat in a good way here. Rubber bumper strips are installed above and at either end of the drive, a pair of small metal leaf springs supports the thin plastic tray the drive is then bolted into, and the bolts are slid into rubber tracks like those used in drawers. Pretty neat, huh? You might be wondering by now what I actually used out of everything I've just discussed. With what I've got to work with, I basically have little choice other than to somehow hang the drive from the roof of my enclosure. The first solution I came up with was extremely simple, and has proved super effective. It involves sinking four screws straight down through a steel plate, with rubber grommets providing cushioning and a nice air gap. That really wasn't so hard, was it?

So, there's some food for thought for you on mounting HDDs. :-)

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