How Does A Hard Drive Work? [Technology Explained]

how does a hard drive workThe average laptop in the shops for around $500 has somewhere in the region of 60GB of memory. You see that figure and think ‘wow – imagine all the movies, songs, images, files and documents I could save on that baby’, right?

But did you ever think about how it actually gets stored?

If you were to stack the equivalent capacity of CDs in front of you it would surely rise to eye-level. You can fit everything on those CDs onto that hard drive. Truly amazing for an invention that has its origins in the 1950’s and was first developed as a humble cassette tape.

How Does a Hard Drive Work – The Basics

how does a hard drive work

In order to fully understand a hard drive you have to know how one works physically. Basically, there are discs, one on top of the other spaced a few millimetres apart. These discs are called platters. Polished to a high mirror shine and incredibly smooth they can hold vast amounts of data.

Next we have the arm. This writes and reads data onto the disc. It stretches out over the platter and moves over it from centre to edge reading and writing data to the platter through its tiny heads which hover just over the platter. The arm, on the average domestic drives can oscillate around 50 times per second. On many high-spec machines and those used for complex calculations this figure can rise into the thousands.

Hard drives use magnetism to store information just like on old cassette tapes. For that reason, copper heads are used as they are easy to magnetise and demagnetise using electricity.

Storage and Operation

hard drive explained

When you save a file, the “˜write’ head on the arm writes the data onto the platter as it spins at high RPM often in the region of 4,000. However, it doesn’t just go anywhere as the computer must be able to locate the file later. It also must not interfere or indeed delete any other information already on the drive.

For this reason, platters are separated into different sectors and tracks. The tracks are the long circular divisions highlighted here in yellow. They are like “˜tracks’ on music records. Then we have the different sectors which are small sections of tracks. There are thousands of these from centre to edge of the platter. One is highlighted blue in the picture.

In Operation

When you open a file, program or really anything on your PC, the hard drive must find it. So let’s say that you open an image. The CPU will tell the hard drive what you’re looking for. The hard drive will spin extremely fast and it will find the image in a nano-second. It will then “˜read’ the image and send it to the CPU. The time it takes to do this is called the “˜read time’. Then the CPU takes over and sends the image on its way to your screen.

Let’s say you edited the image. Well now those changes must be saved. When you click “˜Save‘, all of that information is shot to the CPU which in turn sorts it (processes it) and sends it to the hard drive for storage. The hard drive will spin up and the arm will use its “˜write‘ heads to overwrite the previous image with the new one. Job done.

That is what that buzzing disc in your computer gets up to all day. Now, as I do with most of my articles here on MUO I shall leave you with a friendly word of advice:

If you want to look inside to further understand how does hard drive work, do so with an old one. There are a few reasons for this.

  • Remember when you were a kid and you pulled apart that roll of film with all aunt Lucy’s wedding snaps on it? Well exposing a hard drive to light will damage it and render it as useless as that roll of film.
  • Once you pop open that drive, plugs on the screws will snap to tell the manufacturer you have been poking around in there. By doing this, your warranty is void immediately. Many drives actually have this warning printed on the side.
  • They’re expensive and carry a lot of important info so don’t just pop open the family PC to have a go at it. Pick up an old one on eBay. 3 eBay Tips to Get the Best of Ebay Read More

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  1. Ben R
    October 13, 2016 at 4:53 am

    Well written Explanation!
    Just took one apart to destroy the disc.
    What is the purpose of the super strong pair of magnets one each above & below the disc.?

  2. Hitesh
    August 30, 2009 at 12:03 pm

    Now I am fan of your 'technology explained' section,
    Thank you very much.......

  3. Titanium Pen
    August 26, 2009 at 5:01 am

    That's the problem with the web: people are just so critical. Perhaps the author can explain how you magnets can store stuff...anyway, I learnt something in this article. I didn't know that something in the hard drive is moving.

    Just a question:
    What about USB drives? Any arms or sth like that?

  4. Jerry S
    August 22, 2009 at 7:14 am

    A bit basic... and it raises questions like: how about the different sizes of sectors physically (inside sectors are smaller), compared to the identical sizes in storage (4 k)?

  5. bTuna
    August 21, 2009 at 10:05 pm

    "2.When I refer to 60GB of memory, quite obviously I’m referring to the amount of data that can be stored on the disc."

    Paul G's comment:
    "Most people haven’t a clue about the difference between memory (RAM) and a hard disk.

    A bit disappointing to see the same mistake in an article here."

    Of course, OBVIOUSLY, you weren't referring to RAM. And as Paul pointed out, it's inconceivable that a supposed tech writer would make the mistake of using HD and RAM interchangeable, and that a supposed electrical engineer would defend such usage.

    The "information" offered here has become a joke. Unsubscribing RSS feed in 3... 2... 1.

  6. bTuna
    August 21, 2009 at 10:01 pm

    "(and yes, I believe with the “light” comment he was mistakenly referring to optical disks, CDs, etc..rather than a magnetic hard disk)"

    Yes, because exposure to light will ruin all those optical disks. Be sure to only use your CDs in the dark, kids!

    Seriously, you guys running this place are damn geniuses. "Makeuseof", indeed...

  7. Dean Sherwin
    August 21, 2009 at 1:14 am


    1. Sorry about the last piece of advice about the light. I think I got carried away with the 'roll of film' idea. Basically, when a hard drive is being manufactured they are shined and cleaned until there isn't a trace of dirt, dust or contamination on the disc. Indeed, light for short periods will not damage the platter but any dust in the air or any contamination left behind such as fingerprints or moisture will make it hard for the heads to read the data.

    2.When I refer to 60GB of memory, quite obviously I'm referring to the amount of data that can be stored on the disc. A PC with 60 GB of RAM would be some kind of monster machine ! ;)Paul - I think you'll find most people do know the difference between RAM and hard disc capacity - even on computer advertisments they're labeled in the specs for all to see and understand.

    3. Finally,thank you for the comments and sorry about the light error in the final few lines ;)

    • Rick Stanley
      August 22, 2009 at 10:46 am

      "Light" has NO effect on hard disks AT ALL!!! They even make some hard disk's with a clear cover! ;^)

      Platters are magnetic, NOT light sensitive! It would be the same as saying that a audio cassette is light sensitive.

      PLEASE do your research THOROUGHLY before writing an article like this. Misinformation can be worse than a lack of information. And proper terminology, such as the difference between RAM(memory), and hard drive storage, is important. Most modern disks spin at at least 7200 RPM. There are more misconceptions in the article that I won't comment upon.

      I would also avoid telling anyone to open a hard drive even one they get from eBay. There are enough photos and videos online to satisfy that curiosity.

      • ZZC
        August 22, 2009 at 8:13 pm

        I agree with Rick Stanley's comments.

        The article author also says: "The CPU will tell the hard drive what you’re looking for... It will then ‘read’ the image and send it to the CPU... Then the CPU takes over and sends the image on its way to your screen."

        Also: "When you click ‘Save’, all of that information is shot to the CPU which in turn sorts it (processes it) and sends it to the hard drive for storage. "

        The CPU does all these? The author does not seem to have clear ideas about the operations of a CPU, Operating System and RAM.

  8. SecretsPedia
    August 20, 2009 at 4:08 pm

    nice , now I know from where this sound come

  9. Doc
    August 20, 2009 at 2:57 pm

    @Rich: Right. Hard disks aren't sensitive to light, but ARE sensitive to dust and dirt in the air...which is why the drive is filled with inert gas in a "clean" room (which makes a hospital operating room look like a pigpen) at the manufacturer's facility. (A single particle of dust is big enough to jam between the hard disk head and the platter...which would ruin the drive!) Unlike floppies or audio cassettes, the hard drive heads "float" above the surface of the disk on a cushion of gas.

  10. Albert
    August 20, 2009 at 2:41 pm

    This is the absolute worst article on the operation of a hard drive I have ever seen. It is painfully obvious that the author knows less about hard drive technology than your average 5 year old. In fact this article would insult the intelligence of a computer savvy child. Please, either label this as "satire" or "comedy," or remove it. If you are interested in knowing how a hard drive works check the videos below. They cover hard drive operation, data recovery, and forensics.

    • Ryan Dube
      August 20, 2009 at 3:37 pm

      Albert - as an electrical engineer I would disagree with you. While the article is certainly written "down" for a lay person to understand (and yes, I believe with the "light" comment he was mistakenly referring to optical disks, CDs, etc..rather than a magnetic hard disk), but that was a single error - to call it the worst article isn't fair to the author. The described operation and the diagram is technically correct, albeit simply described.

  11. Rich
    August 20, 2009 at 2:20 pm

    60GB of memory in a PC?!??!?

    Disks sensitive to light??!?!

    You just explained throughout the whole article how the heads magnetically "write" on the disks and then you say that they will be ruined if exposed to light? Is light magnetic?

  12. Drew
    August 20, 2009 at 2:15 pm


  13. Paul G
    August 20, 2009 at 2:13 pm

    Most people haven't a clue about the difference between memory (RAM) and a hard disk.

    A bit disappointing to see the same mistake in an article here.

    • Ryan Dube
      August 20, 2009 at 3:09 pm

      Paul - could you please explain what you're talking about? It appears pretty clear to me that the author was not in any way referring to RAM but was talking about a 60Gb hard drive. What exactly are you referring to?

      • Catfish
        August 21, 2009 at 4:05 am

        He was referring to the fact that, by default, 'memory' is generally considered RAM, while, a hard drive is just called 'hard-drive' or -- 'storage'

  14. anon
    August 20, 2009 at 1:31 pm

    60gb of memory? really? :)