I was always wondering how a hard drive finds the first bit of data.

When a hard drive spins up, whatever it reads must be a circular stream of data until the reading head moves to a different position.

But in such a circular stream, how does the drive know where the first bit and the last bit lie, so that it can pass on the data in the right order?

  • BTW (rarely mentioned but salient) There's an index mark to indicate the start (and end) of the track. When the disk controller encounters the index a second time, then that indicates that it has read every sector in the track.
    – sawdust
    Commented Jun 12, 2017 at 4:51

5 Answers 5


It reads it from the disk.

Data on the disk is not only structured (as @psusi says), but also encoded. The encoding ensures that the recorded data cannot be confused for the position markers in the sector headings, so the circular stream can be read until the target position marker is found.

As I understand it, modern hard drives don't quite do that; they read the entire circle into a buffer, keeping track of where each sector is, and use the buffers to send back requested data.


The magnetic media is a material which has a magnetic field with two key properties: 1) it never changes on its own, and 2) the recording device can change the orientation of the field at any point on the surface. When reading the media, the sensor detects where the field is oriented toward the sensor and where the field is oriented away from the sensor. As the sensor moves across the surface it detects the timings of these polarity transitions; the first layer of decoding is translating these timings into bit values. Due to physically necessary uncertainties in this process, the encoding must not require long stretches of the same polarity; that is, it must be a Run-length limited coding (RLL).

The particulars of hard drive designs are generally trade secrets, but there are essentially two ways to ensure that sector markers never appear in sector content:

  1. Design an RLL that allows special values which will never result from encoding content data. These special values could be used not only for marking sector boundaries but also for error correction or any other secondary purpose.

  2. Use a second layer of encoding that ensures the marker values only appear at the markers. This is a bit like URL encoding to allow special characters to be "hidden" in URLs, but with an additional constraint equivalent to limiting how many characters can be added, so it ends up more like base64 encoding.

So, the read head moves across the surface detecting magnetic polarity changes, the timings of those changes are used to determine the corresponding sequence of bit values (possibly including some exceptional values that don't represent stored data), and that sequence is used to determine which sectors are being read and the content of those sectors. As the content of sectors is determined, the data may be stored in a solid-state buffer and/or stored in a RAM buffer and/or sent back to fulfill a request.

  • Is this encoding like the Huffman coding? If someone could make a clear example of how this works on a hardware level like so: "The head reads a circular data stream like 010111010010111010... where every 111 marks the beginning of a sector, and then..." I could then accept the answer.
    – uzumaki
    Commented Jun 14, 2017 at 22:19
  • Hard drives have to encode abstract data as physical properties of the magnetic media, and the result has to be easily addressable; Huffman coding maps one stream of abstract data to a (usually) shorter stream of abstract data (breaking addressability). They're mostly unrelated. Commented Jun 15, 2017 at 14:19
  • My update didn't add any examples, but if you follow the RLL link it has some. Commented Jun 15, 2017 at 14:21

Data is not written as an arbitrary stream of ones and zeros. It is written in sectors. Each sector has the payload of user data, and a header. The header contains error correcting codes, as well as a special sync field that identifies the start of the sector, and the sector number so the drive can know when it has found the start of a sector, and which sector it is.

  • 3
    Some references or links would make this good answer great. :D
    – cat
    Commented Jun 11, 2017 at 11:36
  • 1
    @cat, I suppose "How computers work" or Peter Norton's "Inside the PC", if either of those are still in print... probably a few new editions since the ones on my bookshelf.
    – psusi
    Commented Jun 11, 2017 at 15:24
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    @MartinArgerami -- I'm not up-to-date on current HDDs (e.g. perpendicular recording), but with MFM recording the address mark consists of a unique magnetic pattern, and the start of a record is sync consisting of two bytes of IIRC 0xA5. "it makes the data storage less efficient" -- Are you too young to remember consumer discontent over the advertised unformatted capacity of ST-506 HDDs versus the usable, formatted capacity (which predates the decimal versus binary MB/GB numbering complaints)? The difference between those two capacities is for those gaps and ID records.
    – sawdust
    Commented Jun 12, 2017 at 4:46
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    @user134593 -- The sync bytes are always after a gap, and are the first bytes of the record. They merely indicate the start of the record. Hence the disk controller never has a conflict or confusion with payload or user data. You seem to think it's a bit stream (just like the OP), but it's magnetic media, which means you cannot just read or write at any point you want.
    – sawdust
    Commented Jun 12, 2017 at 19:35
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    @user134593, it can tell because it is nearly impossible for user data to match the sync field, have a sector ID that makes sense for the track the head is on, and the error detecting codes pass as well. An escape can not be used because that inserts additional data, for which there is no room.
    – psusi
    Commented Jun 13, 2017 at 0:06

Psusi is correct (the data on the disk is structured, and different parts of the computer use different parts of that structure) but doesn't really get at your question.

The drive doesn't really "know" anything. It has low level electronics that can read markers on the disk (generally written at the factory, or by the drive head itself), read data blocks from the disk, or write data blocks to the disk, or tell if a particular spot on the disk is bad or damaged, or that it should move to a particular location on the disk. That's about all it "knows". The reading head doesn't decide to move someplace else by itself, something higher up in the machine tells it to...

  • 2
    But the something higher up is still part of the hard drive. (Which is commanded by something outside the hard drive, which is commanded by something outside the computer, which is commanded by something metaphysical, but we aren't talking about any of those things) Commented Jun 11, 2017 at 12:01

The answer you are looking for has two parts:

1) A hardware controller

2) A file system

Like you said, in a HDD (as opposed to other technologies like SSDs) the actual data is written to round metal plates as concentric circular rings holding a patterned magnetic field. Above the platters that hold this data is the write head which moves around to read and write data, a lot like a vinyl record player. The platters it moves over are attached to an electric motor which controls their rotation.

A hardware controller acts as an interface between the operating system and the hard drive. The controller can read the position of the write head as well as the rotation of the platters and uses this information to decide how to position the head and platters for reading and writing. It translates read and write requests from the operating system into control signals that move the write head and rotate the platters, as well as converts the parallel data coming in from the operating system into a single serial data line. It also splits up this serial line and decides what physical location, or sector, to put each piece in and records this information in a way specified by the file system.

The file system is a specification of how and where to store data. The computer's operating system knows how to interpret this file system and uses this knowledge to adequately communicate with the hardware controller, in this case breaking down the circular rings of data into usable segments called sectors and telling the file system where these sectors are physically located. The file system gives each sector an address, which is just a unique number, and this address gets translated by the hardware controller into a specific platter rotation and read head position to begin reading or writing.

For more information, the following sections in these Wikipedia articles are quite helpful:

See Intro and section 3.1 "Space management" here: https://en.wikipedia.org/wiki/File_system

See section 2.1 "Magnetic Recording" here: https://en.wikipedia.org/wiki/Hard_disk_drive#Magnetic_recording


In addition to the other answers, hard disks certainly used to (and may still do) have one platter ("head" in cylinder/head/sector terms) which is reserved for calibration/positioning data, not used at all for user data storage.

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    This isn't something I've heard of, do you have a reference for it? Commented Jun 11, 2017 at 20:31
  • Yea, no.... that's not a thing.
    – psusi
    Commented Jun 12, 2017 at 0:15
  • You seem to be referring to the servo surface/platter. But that is obsolete technology that I haven't seen since 14" disk packs, which predate the ST-506 HDDs of the original IBM PC-XT. Winchester and modern disk drives use an embedded servo.
    – sawdust
    Commented Jun 12, 2017 at 2:56
  • Ah OK, I haven't heard of this for quite a long time also, although I put it down to modern disks being black boxes. Commented Jun 12, 2017 at 8:58

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