How is data stored in HDD?

The hard disk drive (HDD) was first invented in 1953 by engineers at IBM who were looking for a way to provide random access to high capacities of data storage. HDDs were originally as large as refrigerators, contained 1950’s vacuum tube technology, and had capacities of only a few megabytes. Over time, advancing technologies allowed HDDs to drastically shrink in size while exponentially increasing in capacity. Modern HDDs are 2.5 or 3.5 inches tall and can hold over 10 terabytes of data. While solid state drives are taking over some of the market, HDDs remain a dominant data storage technology due to their high capacity and low cost per gigabyte.

An HDD stores data on quickly rotating platters coated with magnetic material. Read/write heads on actuator arms scan over the platters, magnetizing or sensing the magnetization of the material to read or write data. The platters spin at high speeds, allowing the heads to access data anywhere on the disks within milliseconds. Electronic components coordinate the motion of the heads and transfer of data to and from the platters. HDDs also utilize disk caching to enable faster access to frequently used data.

The key components that enable an HDD to store and retrieve data are: platters, read/write heads, actuator arm, spindle motor, logic board, and cache.

Magnetic Storage

Inside a hard disk drive, data is stored on circular platters made of non-magnetic material, usually aluminum alloy or glass ( These platters are coated with a thin layer of magnetic material, typically a cobalt-based alloy ( As the platters spin, read/write heads float just above them on a thin cushion of air.

Data is stored by magnetizing tiny areas on the platter surfaces. The read/write heads contain electromagnets that can polarize these areas into north or south magnetic orientations, representing binary 1s and 0s. Millions of these magnetic regions represent the data stored on the drive. The strength and polarity of magnetism in each region corresponds to the binary data.

Read/Write Heads

Read/write heads are responsible for reading and writing data to the platters inside the hard disk drive. As described by Disk Structure, read/write heads are mechanical arms that move across the surface of the platters to read or write data. The heads float very close above the platter surface, with clearance typically less than the width of a human hair. This allows the heads to magnetically read and write data to the platter surface.

There is one read/write head per platter surface in the hard disk drive. So for example, a hard drive with two double-sided platters would have a total of four read/write heads, one for each side of the two platters. The heads are affixed on sliders at the end of the actuator arm and work in unison to access the correct location on the correct platter surface when reading or writing data.


Platters are the actual disks inside the hard drive enclosure that store data. They are made of non-magnetic material, usually aluminum or glass. Aluminum platters can withstand rotational speeds of up to 15,000 RPM before risk of shattering.

The platters are coated with a thin layer of magnetic material, typically a cobalt alloy, that allows data to be written and read magnetically. The magnetic coating allows each platter to store data on both sides.

Hard drives contain multiple platters stacked vertically on the spindle motor, with tiny read/write heads for each surface. This allows for increased data storage capacity within the confined space of the hard drive enclosure. The more platters in the hard drive, the more total storage capacity.

It’s important for platters to be made of quality non-magnetic material and have a consistent magnetic coating to ensure reliable long-term data storage. The platter material and construction determines the performance and durability of the hard drive. (Source:

Actuator arm

The actuator arm holds all of the read/write heads and moves them across the platters as needed to access data. It is a pivotal component that enables the hard drive to read and write data across the entire surface of each platter. The actuator arm extends across the platters like a bridge and has a read/write head mounted at the end, one for each platter surface.

When data needs to be accessed, the actuator receives instructions from the logic board about where the data is located. The actuator arm then swiftly and precisely moves the heads to the proper track and sector across the platter surfaces. The arm must move quickly and accurately to access data in milliseconds. According to sources, the actuator arm moves at speeds exceeding 100 miles per hour (1). High precision is also required, with tolerances as small as a few nanometers to position the heads correctly (2). The actuator arm allows the drive to access any part of any platter in the hard disk drive rapidly and reliably.




Spindle Motor

The spindle motor is responsible for rotating the platters inside the hard disk drive. It spins the platters at high speeds, typically between 5400 RPM to 15000 RPM depending on the drive. The spindle motor is a brushless DC electric motor, meaning it converts electrical energy into mechanical energy to rotate the platters.

Higher spindle speeds allow data to be accessed more quickly from the platters, but they also generate more heat and consume more power. Most consumer hard drives have spindle speeds between 5400 RPM and 7200 RPM, while high performance drives can reach 10,000 RPM to 15,000 RPM.

The spindle motor must rotate the platters at a consistent and precise speed. Any variations in speed can disrupt the ability of the read/write heads to accurately access data from the platters. To ensure stability, the spindle motor is mounted on high precision ball bearings and may use a servo control system.

Hard drive spindle motors are designed to operate continuously at high speeds for years. They require specialized engineering to achieve the necessary performance and reliability. Factors like smoothness, low vibration, and handling thermal expansion are critical in their design. Overall, the spindle motor is a crucial component that enables the high performance operation of modern hard disk drives.[]

Logic Board

The logic board is the central processing hub of a hard disk drive. It contains the controller chip and firmware that processes commands from the host computer to read or write data to the drive 1. The controller chip acts like the brain, managing all of the components and operations within the hard drive.

When the host computer sends a request to read or write data, the logic board receives it and communicates with the read/write heads, actuator arm, spindle motor, and other components to execute the command. It controls the flow of electrical current to the various parts in order to position the heads, spin the platters, and transfer data. The logic board ensures everything works together seamlessly.

The firmware on the logic board is low-level software containing the basic instructions for the hard drive. It enables the drive to understand requests from the operating system and perform tasks like initializing the drive, managing data organization, controlling spindle speed, and facilitating data transfers. The firmware optimizes performance and provides a standard interface for the OS.

In summary, the logic board is the central processing unit responsible for controlling all of the hard drive’s components and operations to read and write data as requested by the host computer.


Hard disk drives utilize a small amount of fast memory to cache frequently accessed data in order to improve performance. This is known as disk caching. The cache memory stores copies of data blocks that are read from or written to the platters. Subsequent reads and writes will access the cached data blocks directly from the cache memory rather than having to physically retrieve them from the platters again. This significantly improves the data transfer rate. There are two main caching methods used in HDDs:

  • Read caching stores data that has been recently read. This allows subsequent reads to the same blocks to be served faster from cache.
  • Write caching stores data to be written that hasn’t been physically written to disk yet. The data remains in cache until it can be safely written.

The cache memory is usually DRAM chips on the logic board that can provide nanosecond access times compared to the milliseconds required to access the platters. Typical cache sizes range from 8-256 MB depending on the HDD. Larger caches improve performance but add cost. Advanced caching algorithms are used to determine which data should be cached and flushed based on access patterns [1].

Access Process

The process of locating and retrieving data from a hard disk drive involves several steps that add up to the disk access time. The main components that factor into disk access time are seek time and rotational latency.

Seek time refers to the time it takes for the actuator arm to move the read/write heads to the proper track on the platter where the data is located. The average seek time for consumer HDDs is around 9 ms. Seek time depends on the speed of the actuator motor as well as the distance the heads need to travel across the platters.[]

Rotational latency is the delay waiting for the platter to spin around until the required data rotates under the read/write head. On average, rotational latency for consumer HDDs is around 4.17 ms. After the heads are properly positioned, they must wait for the data to rotate into position under them before reading or writing can happen.

Together, the average seek time and average rotational latency make up the average access time. Access time can vary from around 10-20 ms for consumer HDDs. Data closer to the outside edge of the platter has lower latency than data on the inner tracks. High performance drives focus on reducing access times as much as possible to improve data transfer speeds.


Hard disk drives have been an integral component in computing for decades. The main components that enable data storage on HDDs are the read/write heads, platters, actuator arm, spindle motor, and logic board. The read/write heads are responsible for reading and writing data to the platters. The platters are disks that provide the magnetic surface for storing data. The actuator arm positions the heads over the correct location on the platters to access the data. The spindle motor spins the platters at high speeds. The logic board controls the functions of the drive and manages the flow of data. Caching helps improve performance by storing frequently accessed data.

Since the first commercial hard drive in 1956, HDD capacities have grown exponentially over time, from just a few megabytes initially to multiple terabytes today. This expanding storage capacity has played a key role in enabling our modern digital world by providing a place to reliably store the massive amounts of data we generate and rely on. HDDs continue to be a cost-effective option for large amounts of storage, despite the emergence of solid state drives. The development of hard disk drives has truly revolutionized personal computing and data storage.