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Hard Disk Internal Structure Describe

Hard Disk Structure

As you all know about hard disk and 95% of us have used the hard disk as our storage device. However, it is a big fact that less than 50% of us do not know about the hard disk structure. Now I am going to reveal the internal structure of the HDD and how it works. 

In Short… 

What is a Hard Disk? How does it work? 

A hard disk is a sealed unit containing several platters in a stack. Hard disk (HDD) can be mounted in a horizontal or vertical position. In this description, the hard drive is mounted horizontally.
Electromagnetic read/write heads are located above and below each platter. As the platters spin, the drive heads move toward the center surface and edge. That way, the drive heads can reach the entire surface of each platter. 

Basically structure of Hard Disk(HDD) in Two types:

Hard Disk Structure - Physical and Logical

Let's dig into the subject ...

Physical Structure of HDD

However, we can divide the Hard Disk Physical structure Inti the following terms:

Deviation of a Hard Disk Physical Structure


A platter is a round, metal disk that is mounted inside a hard disk drive. Several plasters are mounted on a fixed spindle motor to create more data storage surfaces in a smaller area. The platter consists of a core containing aluminum or glass substrate, covered with a thin layer of Ferric Oxide or Cobalt Alloy. On both sides of the layer, a thin coating is deposited by a special manufacturing technique. This is the thin cover where the original data is stored at the Media Layer. 

When the magnetic media is applied on the surface of the layer material, a thin oily layer is applied to protect the material. These complex three-level media are discussed in detail: 

HDD Physical Structure - Platter Deviation


The platform builds on the grounds on which a large amount of media layers can be deposited. Apart from the media layer support, the substrate has no specific function. The most commonly used material for making this physical layer is a mixture of aluminum. This blend is rigid, lightweight, stable, inexpensive, efficient and readily available. Earlier, since the spacing between the head and the plate was relatively high, the surface of the blade was smooth and the plane was less of a problem. However, with the advancement of technology, the gap between the head and the platters is decreasing, and the speed at which the platters spin is increasing. Because of this, the demand for alternatives to platter material is increasing. Glass platters are replacing aluminum platters because they provide improved rigidity, better quality, thinner platters, and thermal durability.


Substrate elements form the basis on which the actual recording media is deposited. The media layer is a thin coating of magnetic material that is applied to the surface of the platters and where the actual data is stored. Its thickness is only a few millionths of an inch.
Special techniques for the deposition of magnetic materials are employed in the substrate components. A thin coating is deposited on both sides of the layer, mostly called magnetron sputtering by the vacuum deposition process. Another such method is electronic compression, using the same process as electronic connection jewelry.


A super-thin, protective, lubricating layer is applied to the top of the magnetic media. This layer is called the protective layer because it protects the disks from damage due to accidental contact to the head, "head crash" or other foreign elements entering the drive.



To maintain organized storage and data retrieval, platters are organized into specific structures. These specific structures include tracks, sectors, and clusters. Let's briefly explain ...


Hard Disk Structure
HDD Tracks | Sectors

Each platter is divided into thousands of tightly packed concentric circles, known as tracks. These tracks are similar to the structure of the tree's annual ring. All data stored on the hard disk is recorded on the tracks. Starting from zero on the outside of the platter, the number of tracks is increasing inward. Each track can count large amounts of data in thousands of bytes.


Each track is further broken down into smaller units called sectors. As the sector is the basic unit of data storage on an HDD. A single-track typically can have thousands of sectors and each sector can hold more than 512 bytes of data. A few additional bytes are required for control structures and error detection and correction.


Each partition on your hard disk is divided into clusters. A cluster is the smallest possible unit of storage on a hard disk. The size of a cluster depends on two things:
  1. The size of the partition.
  2. The file system installed on the partition.
Sectors are often grouped together to form Clusters.



HDD Structure - Read/Write Head
HDD Read/Write Head | Image: Wikipedia

The heads are the interface between magnetic media where data is stored and the electronic component on the hard disk. Converts the bits into magnetic pulses in bit size while storing the heads on the platter and reverses the process while reading. The heads are the most sophisticated part of the hard disk. Each platter has two read/write heads, one at the top and the other at the bottom. These heads are mounted on the head slider, which hangs at the end of the head arms. The arms are all joined together in a single structure called actuators, which are responsible for their movement.



HDD Physical Structure - Spindle Motor
HDD Spindle Motor

Spindle motors play an important role in hard drive operation by turning hard disk platters. A spindle motor must provide stable, reliable and continuous turning power for continued use for several hours. Many hard drive failures occur because the spindle motor is not working properly.



Hard Disk Physical Structure - Logic Board
Hard Disk Logic Board

Hard disks are made with an intelligent circuit board integrated into the hard disk unit. It is mounted on the outside of the base casting exposed on the outside. The read/write heads are connected to the logic board via a flexible ribbon wire.



A Hard Disk drive (HDD) has a logical structure that is compatible with the operating system installed. The master boot record is the most important part of the hard disk. This is the first sector on the hard disk that the operating system accesses. The master boot record contains boot loaders and partition tables. It is important to know the logical structure of the hard disk to better understand and resolve the hard disk drive problems.
Basically, we can divide the hard disk logical structure into the following five logical terms:


There is a small program to load and start active (or bootable) partitions from hard disk drives, known as master boot records (MBRs) or sometimes master partition tables (MPTs). The master boot record contains information about the four primary partition types of the hard drive (s), such as startup area, completion area, partition size, and so on.
Hard Disk Logical Structure - MBR
MBR - Master Boot Record

The MBR is located in Sector 0 or we can say in Cylinder 0, Head 0, and Sector 1, and if there are multiple partitions on the disk, then there is Extended Master Boot Records located at the beginning of the volume for each extended partition (see the following figure).
MBR is created on the hard disk drive by executing the DOS command - FDISK.EXE. However, there is more software available to do the same. Anyone of these partitions can be enabled or bootable using FDISK. This allows you to control the boot sector of the active partition when the system is started. Since the floppy has no partition and no MBR.

  • At the beginning of the hard drive is the MBR. When your computer starts using your hard drive(HDD), this is where it looks first. 
  • The MBR itself has a specific organization. The size of the MBR is 512 bytes. 
  • The bootloader is the first 446 bytes of the MBR. This section contains executable code, where programs are housed. 
  • The partition tables are 4 slots of 16 bytes each, containing the description of a partition (primary or extended) on the disk.

You can get a clear idea, how to describe a partition:

1.State of the partition (inactive partition bootable) - (1 byte)

2.Custom heads at the beginning of the partition - (1 byte)

3.Cylinder sector and the beginning of the partition - (2 bytes)

4.Type of partition (file system, eg, 32 fat, etc ...) - (1 byte)

5.Head of the end of the partition (1 byte)

6.Cylinder sector and the end of the score - (2 bytes)

7.Number of sectors between the MBR and the first sector of the partition - (4 bytes)

8.Number of Sector of the partition - (4 bytes)


After the power-on self-test (POST), the BIOS loads from the hard disk into MBR (Master Boot Record) memory and later performs it. First MBR tests the hard disk for an active partition, then loads it into the DOS boot record (DBR) memory and removes control over the operating system boot code, and then loads the rest of the operating system boot record code into the memory.

NB: If I describe all the topics, this article goes a long way. So I suggest you do a little research on Wikipedia, you can get a broad knowledge of this topic. Also, I give here an overview of the rest of my article.


Full form - File Allocation Tables, FAT is a method for tracking the contents of hard drives used by Microsoft operating systems, first launched in 1977. The Table is a chart of numbers that matches the cluster address of a hard drive. The following is a list of the different types of FAT used and the operating system using them.
  • FAT 8
  • FAT 12
  • FAT 16
  • FAT 16B
  • FAT 32
  • ExFAT
  • FATX
  • FAT+
To be continued in our next publishing...

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