How It Works: Hard Disks

How It Works: Hard Disks

by Dustin Sklavos

It seem that the more exciting parts of the How it Works series are behind us. We’ve covered the graphics, memory, and processor – three key performance-defining elements of modern computers in general. While there’s no question that these components are the big performance workhorses of the machine, it bears mentioning that the memory and northbridge are designed with mitigating the low bandwidth of the hard disk in mind.

Even beyond simple performance characteristics, understanding the hard disk in your notebook is very important. After all, it’s where you keep all of your data. Family photos, video games, school work, music…all of it is being stored here. In some cases your hard disk can quickly become even more important than your car. You can always get a ride somewhere, but how easy is it to recover lost memories?

How It Workd: Hard Disks

As mentioned in the introduction, the hard disk is basically where everything on your notebook is stored to be accessed later. It’s also the second slowest component in your notebook in terms of bandwidth (the slowest being your CD/DVD drive). Since we love our analogies here at How it Works, you can think of the hard disk as being the bank, and the memory (as mentioned in Part IV) as your wallet. The bank stores all of your important stuff, but in order to get it out, you have to actually go there. Your wallet holds a lot less, but you’ve always got it on you. And much as the bank tries to maximize convenience by offering more branches and adding debit cards, hard disk manufacturers have also made strides in improving access speeds and performance characteristics.

Before we move on, it bears mentioning that the terms hard disk and hard drive mean essentially the same thing. They’re interchangeable. For the purposes of this article we’re going to talk about the mechanical hard disks that 99% of us are familiar with. I’ll do a small blurb about solid state disks at the end, but for now it’s not really relevant.

In this article, I’m going to first explain how the hard disk physically works so you can understand all of the specs that follow it: form factor, spindle speed, cache, interface, and capacity. At the end, as I said, we’ll talk about solid state disks, and I’ll make some recommendations and talk about brands a bit.

The Hardware Itself

Hard disks are actually pretty interesting pieces of hardware. If you were to open one (which you should never do under any circumstances short of being a licensed technician in a clean room), you’d see a series of metallic discs (as many as three in a notebook drive), and mechanical heads hovering over them.

A hard disk stores data on these discs, called platters, and this data is accessed through those mechanical heads, called drive heads. The software within the hard disk itself, called firmware, operates these parts and optimizes how they function to maximize performance and reduce this troublesome bottleneck. The drive itself also contains a small amount of its own dedicated memory (non-upgradeable) called a cache that also helps improve performance, but more on that later.

Given that a drive head has to physically move over a spinning platter to get your data, it should come as no surprise that the hard disk is one of the slowest components in the system. In order to read your stuff, something has to actually physically move someplace else.

The key component here is the platter. The more densely packed data is on the platter, the shorter distance the drive head has to move to get to it, and thus the faster the disk can operate. This results in an aspect that at first seem counterintuitive: the greater the capacity (more on this later), the faster the disk.

Now, remember how I mentioned that you shouldn’t open the hard disk unless you were a licensed technician operating in a clean room? The reason for this is because of how ridiculously fast the platters spin and the drive head moves. A single particle of dust on a platter can fatally damage a hard disk. Alternatively, if the drive head fails, it can collide with the platter and ruin the disk, which is where you get the term crash. The disks are designed to be as robust as possible, but the hardware within is still somewhat fragile, so violently shaking your laptop while you’re copying files is probably not a good idea.

You don’t hear about bad memory that often, and almost never defective processors, but everyone hears about hard disks crashing. Manufacturers try to make them as reliable as possible and are standardizing on five year warranties, but there’s only so much you can do with a mechanical device like this. I’m not trying to make you paranoid like a mental health student who suddenly thinks he’s a paranoid schizophrenic hypochondriac, because these things really are pretty reliable (otherwise they wouldn’t have flourished the way they have). Hardware can still fail, though.

So how can you know if your hard disk is becoming bored with life? Hard disk failures in my experience have never been immediate, sudden things. There’s usually a warning sign affectionately referred to as the click of death. While your hard disk is usually operating, the little hard disk light on your laptop (typically symbolized by a cylinder) is blinking and you hear faint, asynchronous crunching. The click of death is not dissimilar to the sound a CD player makes when it’s trying valiantly to read a CD you’ve left on the floor of your car a little too long: it keeps making the same series of noises at the same speed over and over again before finally informing you that CD cases were invented for a reason. The click of death will be a single hard click that often follows the typical crunching sound a hard disk makes, but it will repeat the exact pattern over and over again. In the process, your computer will suddenly become completely unresponsive in trying to open anything stored on that drive. Eventually, if you ride it out you be able to back up your stuff. Odds are you’re screwed, though.

So how does all this stuff work? (I know, great lead-in.)

Form Factor

First, the important thing to know is that a desktop hard disk isn’t going to fit in your laptop. This probably seems obvious to some of you, but it bears mentioning. There are three common form factors, or sizes, of hard disk.

3.5″ drives are used in desktop computers. These are large, heavy SOBs that as of the time of this writing can hold up to 1.5 terabytes. They draw the most power, and will not fit in your laptop.

2.5″ drives are the most commonly used in laptops. These offer a good blend of performance and capacity without being too heavy, drawing too much power, or taking up too much space. These are the drives we’re talking about in this article.

1.8″ drives are the most commonly used in hard disk based media players as well as high end (read: expensive) ultraportable laptops, not to be confused with netbooks. These tend to have mediocre performance and the lowest capacity, but they’re also tiny and keep power consumption and weight down.

The hard disk in a notebook is generally user-replaceable and it’s pretty easy to do, but there’s a caveat: your data isn’t going to magically hop aboard this new one, so you’ll need to back it up first and then use your recovery media (you did write it when you got your laptop, right? ) to reinstall your operating system.


I know, I’m handling all the exciting stuff first, right? This is important, though. If you’ve bought your laptop in the last couple years, it probably uses Serial ATA, or SATA to connect to the computer. Serial ATA is the standard all modern laptops use, as it’s the fastest and more importantly, universal between desktops and notebooks. Yes, this means you can internally connect a notebook hard disk to a modern desktop. SATA is characterized by two L-shaped plugs next to each other on the back of the drive.

Parallel ATA, or PATA, and originally known as IDE (or EIDE) is the old standard, and many laptop hard disks available in retail use this standard. PATA is identifiable by two rows of pins on the back of the drive, and is generally slower than SATA. PATA laptop disks require a special converter to connect to desktops.

This is important to know if you’re planning on upgrading the hard disk in your laptop, so if you’re unsure about which interface your laptop uses, check the notebook’s specifications on its manufacturer’s website.

Spindle Speed

Much like a vinyl, platters inside the hard disk spin. But while your mint Culture Club record rotates at 45 revolutions per minute, or RPM, the platters inside a hard disk can spin as fast as 7,200 RPM. On desktops drives this speed can hit 10,000 RPM, and server disks can reach a scorching 15,000 RPM. And of course, the faster the platters in the drive rotate, the quicker data can be retrieved, but the hotter the drive runs and the more power it draws. This is largely why notebook drives top out at 7,200 RPM and typically ship at 5,400 RPM.

That said, thermal characteristics and power draw can change depending on the manufacturer and the kinds of optimizations they’ve made to the hardware. While in theory a 7,200 RPM drive should draw much more power than a 5,400 RPM due to the increased draw from spinning the motor that much faster, this is offset by the faster accesses from the 7,200 RPM drive. Simply put, it doesn’t have to spin as long to get your data, so the power draw tends to even out.

Of course, that doesn’t change the heat dissipated by the drive. Depending on where the hard drive is located inside your notebook, this could be either a non-issue or a serious problem. I had an old Gateway laptop that practically buried the drive on the bottom of the notebook, so it wasn’t an issue when I upgraded to a 7,200 RPM drive. On the flipside, my current notebook, a custom-built (and fantastic) HP dv2500t, places the hard drive squarely under the left palm rest. After a little while, the thing gets so hot it makes your palm sweat. I had to stick to a 5,400 RPM drive, much to my chagrin. Your mileage vary.


In order to improve performance, hard drives also contain their own tiny amount of memory, or cache. This is really a sort of non-issue that you shouldn’t spend much time thinking about. Notebook hard drives tend to come with between 8MB and 32MB of cache, averaging out at 16MB. While cache can have a definite impact on how a hard disk performs, the optimizations the manufacturer has made in how it’s used tend to have a greater one. As a result, most retailers won’t even tell you how much cache the drive has as it’s generally irrelevant.


And here we have the reason why hard disks haven’t been replaced. Simply put, there isn’t a better, more cost-effective technology for storing massive amounts of data on the market. We’ve already discussed how the data is stored and how it’s accessed, and way back in Part I we defined how to measure capacity, so for the most part we should be good to go, right?

Mostly. There are a couple of key elements here. First, while I said that a larger drive will oftentimes be faster than a smaller one (assuming the same spindle speed), this does get a little bit tricky. For example, if a 100GB drive has three platters of 33GB apiece, this drive will generally be a bit slower than an 80GB drive with two 40GB platters. However, by this logic, you can assume that the largest drives on the market will be among the fastest at their spindle speed.

Second, there’s the way GB are supposed to be measured and the way hard disk manufacturers measure them, and this is something I alluded to in Part I. Simply put, a drive advertised with a capacity of 100GB will show in the neighborhood of 90-94GB of space in the operating system. This is normal; your operating system measures bytes in base 2 as opposed to base 10 which results in this disparity.

You should keep in mind that capacity is also further reduced by the installation of the operating system and by the file system that operating system uses. File systems aren’t worth going into, but suffice to say your operating system needs some way of knowing where everything is on the drive and the file system covers this. Capacity can be reduced yet again by the manufacturer storing the recovery partition on the drive itself. After all is said and done, your shiny new notebook with a 160GB hard drive only be offering you in the neighborhood of 90GB free.

Worth mentioning is that while some larger notebooks include more than one hard disk bay, the vast majority do not, so the capacity of the single drive in your notebook is what you’re stuck with.

Solid State Society

Mechanical hard drives are the standard in modern computing, and this makes them cheap to produce en masse. However, the next great technology is upon us: solid state drives, or SSDs. These drives occupy the same space that your existing hard drive does and is even backwards compatible, but what changes is how the drive stores its data.

A solid state drive uses flash memory, like what you use in your digital camera or in a USB thumb drive, only it uses higher quality flash memory and it uses a lot more of it. Theoretically, this radically reduces power consumption and heat dissipation while dramatically improving acess times.

SSDs are a young technology, however, and while the industry is ramping up to produce more flash memory (and indeed, flash prices have been falling like rocks as a result), the drives aren’t quite ready for primetime. Prices for SSDs are exorbitant, capacity is mediocre, and transfer speeds are in some cases poorer than their mechanical counterparts. This technology will mature and take over for mechanical drives, but it’s a ways off still. As it stands, they generally aren’t worth the money for any but the most indulgent buyers.

As a side note here, it bears mentioning that the burgeoning netbook market is thick with SSDs, but there are caveats here. These drives are typically slower than standard mechanical drives and offer a fraction of the capacity. These are included because a few chips of slower flash memory are easier to fit into a small netbook chassis, draw minimal power, and are less expensive.


As far as drive recommendations go, it gets a little bit trickier. I’ve found that drive reliability in modern computers is worlds more consistent than it has been in the past. I’ve seen drives from every major brand fail (excepting Fujitsu and Toshiba, both of which are often too slow to be relevant anyhow). Manufacturers typically employ Fujitsu drives on the low end, with Hitachi, Seagate, and Western Digital drives filling out the rest.

In my experience, Hitachi drives have been the fastest while running the hottest. This seems to be the prevailing theme over most (not all) of their products. I’ve used Hitachi drives before and been immensely satisfied by the performance, but the heat dissipation off their 200GB 7,200-RPM drive was too much for my HP dv2500t.

Western Digital’s drives fill out the market nicely. They’re solid performers and don’t dissipate too much heat. Though they have what is in my opinion the best price/performance knockout on the desktop scene, the rest of their lineup is just solid. Nothing wrong with Western Digital, but nothing performance leading either. The 320GB 5,400-RPM drive I use in my notebook isn’t the fastest thing in the world, but it does approach the performance of 7,200-RPM drives, and be an excellent compromise for speed, power, heat, and capacity.

Seagate is all over the map. I’ve always loved Seagate drives and found them to be exceptionally reliable, but their products range from industry leading to middle of the road. You can’t really go wrong with a Seagate, and they were offering a five year warranty well before the other manufacturers.

Last is Samsung. Samsung has a pretty devoted following among our forums, and I can understand why. While Hitachi, Western Digital, and Seagate duke it out for the limelight, Samsung offers hard disks at lower prices than their competitors while oftentimes matching them for performance. I don’t have a great deal of experience with their notebook drives, but found their desktop drives to be a good value.

Now, let’s do what we do and condense it all. First and foremost, the most important specs for you are going to be capacity and spindle speed. Interface is good to know for those itching to upgrade, while form factor is largely a given and cache isn’t worth the effort of researching.

The rest of your distillation:

  • Higher spindle speed generally means a faster, hotter drive.
  • Capacity in Windows (or Linux, or Mac OS X) will always read as less than advertised.
  • Solid state drives are the future, but not worth worrying about right now.
  • If you’re going to upgrade your hard disk, back up your stuff, have your recovery media ready, and make sure you buy a drive with the right interface.

Coming Up:

So I’ve told you how stuff gets stored on your computer, but how do you put stuff on it? Optical drives!






Leave a Reply