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How Hard Drives Work

How Hard Drives Work

Most people have thousands upon thousands of megabytes of data on their computer. Indeed, the hard drives we use today to store all that information have grown by astronomical amounts since the early days. Have you ever wondered just how a hard drive actually works? Before we touch on how data is actually read and written to the disk, it’s important to cover the basic elements of a hard drive. We will be primarily covering today’s platter-based hard drives and briefly cover the new solid-state disks (SSDs) you may have heard about as well.

In today’s hard disks, there are seven basic elements to their composition: The enclosure, spindle, platter, motor, actuator arm, interface and logic board. The enclosure is the most simple of the seven pieces, being nothing more than the shell to which everything is installed or bolted. The enclosure houses the internal components (spindle, motor, actuator, platter) and carries the logic board on the underside. All this is completed with an interface on the back.

Hard drive enclosure

The enclosure of a Hitachi Desktar hard drive

The Logic Board

Turning the hard drive over reveals the logic board. This is an extremely important piece that handles several important elements. First and foremost, the logic is the instruction manual for the computer attempting to access the drive. This can be called the detection routine, and serves to give the computer an idea of what the drive is, how big it is, what cable it’s connected to and how to access the drive in your OS of choice. With a dead logic, your PC may never detect that you even have a hard drive installed in the PC, much less its capacity or model!

In addition to the detection routine, the logic board holds the read/write cache, which is crucial to the performance of hard disks. If you tell your computer to open 1000 megabytes of information, the hard drive passes the information to you as quickly as it is able. While the hard drive is loading the first 16MB of the file, the next chunk of data is prepared to roll and is waiting in the cache; when you open the cached chunk of data, another is fed into the cache, and so on until all the information is opened. The reverse of this process occurs when writing information as well. This procedure ensures that data retrieval/archival is expedient, overcoming some of the limitations of a hard drive’s inherently-sluggish, mechanical design.

Lastly, the hard drive’s logic board translates the computer’s requests for archival/retrieval into the commands that make the hard drive do its magic and actually read/write information. Logic boards often have more powerful processing units and more memory than computers of the early 1990s!

For more information on logic boards, StorageReview has a fantastic primer on its very complicated function.

Logic board

This underside of a PCB; all the ICs and cache are tucked against the enclosure

The Spindle Motor

Moving to the internal components, the most basic component therein is the spindle motor. Very precisely-controlled by the logic board, the spindle motor is what rotates the platters, or the disks that store data inside the hard drive. For your average desktop hard drive, the spindle hurtles the platters along at 7,200 or 10,000 RPM. In a notebook hard drive, the rotational velocity is usually 4200 or 5400 RPM. Higher rotational velocities can significantly decrease data read and write time, as the location of the data on the platters can be sought more quickly. The spindle motor is connected to the spindle, which holds the platters centered and stable for the high rotational velocities.

Hard Drive Platters

The platters themselves are probably the most important and complicated component. Today’s platters are thin disks (Thus the “Hard disk” name) of glass or aluminum, coated with an ultra-thin layer of a cobalt alloy, which is naturally magnetic. Data is written to sectors which are organized into concentric rings outwards from the spindle called tracks, and all of those are managed into clusters by the file system you’ve chosen. To actually write the data, the actuator arm aligns the magnetization of the platter in a pattern recognizable to the hard drive’s logic board.

Today’s hard drives contain multiple platters, but the real Holy Grail of hard drive technology is the areal density. The areal density is a term that describes how many sectors fit in a given track (Or in², as a standard unit). The benefits of an increased areal density are two-fold. First, a single platter can contain more information, thusly increasing the capacity of a hard drive. Secondly, and more importantly, when the areal density is higher, the actuator arm that is responsible for physically moving to read/write information can move a shorter distance, thereby increasing sustained speeds in long periods of read/write activity.

A new technology called perpendicular recording aligns the sectors on a platter perpendicular to the physical platter, allowing drive manufacturers to shorten the space between sectors, increasing areal density remarkably while improving drive reliability. Older hard drives align sectors horizontally with the platter, wasting precious space platter surface area. Today’s perpendicular storage drives are approximately 180Gbit/in², while technologies in development, such as IBM’s Millipede, technology is at 1Tb/in² but is commercially unavailable.


A hard drive platter being accessed by an actuator arm.

Platter structure

A) Track, B) Sector track, C) Sector, D) Cluster – Image via: Wikipedia

The actuator arm is the last internal component explicitly responsible for the read/write process, and its job is very simple. Moved to and fro by the activity of two very strong rare earth magnets, two different tips on the actuator can sense the magnetization patterns to read information, and alter magnetization to write information. The actuator’s function is controlled by the logic board, as are all the other internal functions.

Reading/Writing to Disk

In the platter diagram, the green stripe is a cluster that we touched on briefly. A cluster is a collection of sectors grouped together by the file system for more simplified, but not necessarily efficient access. By virtue of clustering, some of the potential storage space on a hard drive can be lost. For example, if a hard drive is formatted with a 4k cluster size, and you write a 2k file to disk, half of that cluster’s storage space is used up by data, and the other half is empty, wasted space. Yet without clustering, disk performance would be very sluggish due to the file system’s inability to access data quickly. Millions of clusters is better than billions of sectors in the long run!

Let’s quickly step through the process of a read/write!

  1. User requests information on the hard drive.
  2. Operating system accesses the MFT, or master file table (An index of files and locations), via the motherboard’s hard drive controller to find the file’s cluster.
  3. Operating system tells the hard drive’s logic board, via the hard drive controller, that it wants a file from a cluster.
  4. The logic board spins up the platters on the spindle.
  5. The actuator arm is moved into position.
  6. The logic board reads and amplifies the very weak, isolated magnetic fields that comprise your data.
  7. The logic board begins using the actuator to read information from the sectors in the requested cluster.
  8. Information is streamed into the hard drive cache
  9. The information is fed from the cache, to the hard drive controller, to you and your RAM!

The write process is almost the exact opposite, except instead of accessing the MFT to find a file’s location, it’s accessing the file table to find free clusters for write space.

Solid State Hard Disks (SSD)

Unlike conventional platters which are, when compared to flash memory, extremely slow due to their mechanical nature, solid-state disks use NAND flash memory to store information. SSDs feature very fast burst read/write speeds, which are periods of disk activity that are much faster than normal due to data being physically close and easy to recover. On the other hand, they are slower than today’s hard drives for writing sequential clusters of information, a process known as sequential writes. Though, due to the nature of a user’s interaction with their PC, a majority of the accessing done on a hard drive is data reading, which SSDs are twice (Or more) as fast at as conventional hard disks.

Also, due to a complete absence of moving parts, SSDs have greatly increased longevity and reliability. The downside to these drives, despite all their amazing benefits, are very high cost per megabyte of capacity, and their low capacity. It is unfortunate that both these traits are what will really stop a consumer from adopting a technology that is superior. There can be no doubt that SSD is the future, but that future is assuredly not now. However, in light of the low capacities and high prices, the technology still has enough merit to be featured in new notebooks arriving in 2007. Reduced power consumption, reduced heat and a long battery life spells great success for this technology in the mobile arena.


There you have it! The seven essential elements of hard drives, and their most likely successor. While SSDs certainly sound promising, don’t think that conventional hard drives are dead. Major manufacturers like Seagate and Western Digital are feverishly dumping millions of R&D into new hard drive technology to further increase the capacity of our retail hard disks. With Hitachi’s 1TB drive, the world’s first, just around the corner, you can rest assured that the war is far from over, and the good old platter isn’t going anywhere any time soon.


  1. primesuspect
    primesuspect FANTASTIC article, Rob. Great work :)
  2. Sledgehammer70
    Sledgehammer70 Sweet! a place to point people to who ask... "How do Hard Drives work?"

    Great read Thrax :)
  3. Cyclonite
  4. Winga
    Winga Great job Thrax. Even I understood it :D
  5. osaddict
    osaddict That was cool thanks - a brief summary but still with depth! :D
  6. docchris
    docchris From the section about the spindle motor:

    "Higher rotational velocities can significantly increase data read and write time"

    Surely it either increases read/write SPEED, or DECREASES the time taken
  7. Thrax
    Thrax Yes, that's correct. Thank you for pointing out the error. :) It has been corrected.
  8. rice-burner
    rice-burner Thanks for the hard drive technical review.

    I have a question regarding spindle speed vs areal density.

    Is it true that the maximum data rate read from the platter to the buffer (and visa versa) by a single read head, is proportional to SPINDLE SPEED x LINEAR TRACK DENSITY.

    So potentially the figure of performance for a disk drive is SPINDLE SPEED x SQUARE ROOT(AREAL DENSITY).

    In another words: A disk with twice the spindle speed is twice as good as the same disk with double the areal density instead.

  9. Thrax
    Thrax It is true, however as the spindle RPMs increase, the track density must decrease due to reliability concerns.
  10. rice-burner
    rice-burner Rice Burner said:

    So potentially the figure of performance for a disk drive is SPINDLE SPEED x SQUARE ROOT(AREAL DENSITY).

    Now ... Correcting my Math in conclusion ... Doh:

    In another words: A disk with twice the spindle speed should be equivalent in performance to the same disk with four times the areal density instead.

    Thrax replied:

    It is true, however as the spindle RPMs increase, the track density must decrease due to reliability concerns.

    Now thats straight. Thanks.
  11. Medlock
    Medlock A good read. Nice article, Thrax. :)
  12. Naveed
    Naveed actually i have a harddisk whose actuator arms is continusoly moving to and fro. can any 1 resolve this probelm my e-mail id is munaveed2002@yahoo.com
  13. Confused
    Confused What happens if the hard drive has more than one platter? Is data written to one platter then the next when the previous is filled. Or is the data spit up and written to all the platters at the same time making it faster than single platter disks.
  14. Thrax
    Thrax Hi, Confused. :)

    Hard drives put data wherever there's a physical space on any of the platters that's large enough to hold the binary in question. Once the data is broken down into small chunks called blocks, the file system (like FAT32 or NTFS) records where those blocks have been placed, and should they get shuffled around, where they have gone.

    It's easiest to imagine that a hard drive is a big filing cabinet with a master index on the side that lists where every page of every document is stored. A hard drive might not put all the pages in the same drawer, but instead you can read the index and see what drawer each page is saved in.

    A hard drive will attempt to store the entirety of a file on a single platter (it's faster this way), but it won't hesitate to store it on multiple platters if the need arises. As far as I am aware, a hard drive can only read from one armature at a time, though I'd be happy to see if I could get you an official answer from a hard drive manufacturer.

    The biggest speed boost for a mechanical drive comes from increasing the drive's areal density, or how many bits of data can be stored per square inch. The higher the density, the less distance an armature has to cover to load an entire file.
  15. Davey Jay
    Davey Jay Thanks, this helped quite a lot when I had to reverse engineer one of these meanies (meanies because it cut me pretty well when I was fighting it open - rushed for time, had to get physical). Thanks!
  16. cheer
    cheer cheers for this, helped me lots with my ICT asignment :)

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