What Is a Hard Disk Drive (HDD)? Example Calculation & History

Computers rely on storage to operate efficiently, and memory is crucial for processing data. Primary memory like RAM (Random Access Memory) plays a vital role but is limited in size and volatile, meaning data is lost when the power supply is interrupted.

To overcome these limitations, secondary memory provides large and permanent storage solutions. Among the many types of secondary memory, hard disk drives (HDD) stand as one of the most essential components for long-term data retention.

HDDs are fixed storage devices integrated into computers, designed to retain data even when the system is powered off.

These drives are indispensable for storing operating systems, applications, and files. In contrast to primary memory, HDDs offer significant storage capacity and permanence, making them invaluable for modern computing.

In addition to fixed storage devices, secondary storage can also include removable devices like USB drives and external HDDs that offer further flexibility.

This article explores the anatomy, working mechanism, advantages, and evolving role of HDDs in computing technology.

What Is a Hard Disk Drive (HDD)?

A Hard Disk Drive (HDD) is a storage device that employs magnetic technology to store and retrieve digital data.

At its core, the HDD is composed of spinning platters coated with magnetic material, and a read/write (R-W) head that moves to access specific data.

The platter is divided into circular tracks, which are further subdivided into sectors. Although the outer tracks are larger than the inner ones, both contain the same number of sectors.

Surprisingly, this is due to the higher density of data on the inner tracks where bits are packed more tightly.

However, a small portion of each sector is reserved for formatting, resulting in slightly less usable storage than the stated drive capacity.

To locate and retrieve specific data, the R-W head needs to correctly position itself over the spinning platter. Key terms associated with HDD operations include:

Seek Time: The time the R-W head takes to move from its current position to the desired track.
Rotational Latency: The time required for the spinning platter to position the desired sector under the R-W head.
Data Transfer Time: The time needed to transfer the requested data. This depends on the disk's rotational speed.
Controller Time: The processing time taken by the HDD controller.
Average Access Time: Calculated as the sum of seek time, average rotational latency, data transfer time, and controller time.

Rotational latency is typically half the rotational time of the platter because the desired sector may be just beneath the R-W head or require up to one full rotation to align.

Example Calculation

To clarify the concept, here's an example of this hardware:

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A hard disk has 4 surfaces, 64 tracks/surface, 128 sectors/track, and 256 bytes/sector.
The total disk capacity can be calculated as:
Disk Capacity = surfaces × tracks per surface × sectors per track × bytes per sector
= 4 × 64 × 128 × 256
= 8 MB

Next, if the disk operates at 3600 RPM, we can determine the data transfer rate as:

In one second, there are 60 rotations.
Data transfer rate = number of rotations per second × track capacity × number of surfaces.
= 60 × 128 × 256 × 4
= 7.5 MB/sec

Finally, we calculate the average access time. Given that seek time and controller time are not provided, and no data transfer is noted, average access time equals average rotational latency.

Rotational latency:
= (1/60 seconds per rotation)
= 16.67 milliseconds
Average rotational latency:
= (16.67/2) milliseconds = 8.33 milliseconds

History of HDDs

The development of hard disk drives revolutionized computer storage. A timeline of major innovations includes:

1953: IBM began developing low-cost random-access storage solutions.
1956: IBM released its first HDD, the IBM 305 RAMAC, with a 3.75 MB capacity and a size comparable to a refrigerator.
1960s-1970s: Industry players like Memorex, Seagate, and Western Digital entered the HDD market.
1980s: HDD sizes of 2.5 and 3.5 inches were introduced and standardized for personal computers.
2007: Hitachi launched the first 1 TB HDD.
2015: The first 10 TB HDD was introduced by HGST (now a Western Digital brand).
2021: Western Digital unveiled 20 TB drives for enterprise environments.
2025: HDD technology advanced to offer capacities up to 36 TB leveraging HAMR and MAMR technologies.

Why Do Computers Need HDDs?

HDDs are essential for storing operating systems, applications, and user-generated data such as documents and media files. Unlike volatile memory, they retain information even after the computer is powered off.

Without HDDs or equivalent secondary memory, users would be unable to save their work or access critical software needed for everyday tasks.

How Do HDDs Work?

A typical HDD consists of several disk platters mounted on a spindle, enclosed within a sealed chamber. These platters rotate at high speeds, ranging from 5,400 to 15,000 RPM.

The R-W heads move across the platters to retrieve or store data by reading and writing magnetic signals. Finally, the disk controller processes the data and transfers it to the computer system.

Storage Capacities of HDDs

Storage capacities vary widely across HDDs, catering to diverse needs. Common categories include:

Low Range: 16 GB to 64 GB (found in older devices).
Entry-Level: 120 GB to 256 GB (suitable for basic systems).
Mid-Range: 500 GB to 2 TB (often sufficient for average users).
High-End: Over 2 TB (ideal for professionals working with large files, such as high-resolution media).

Modern HDDs are available with capacities up to 20 TB or more.

Features of HDDs

Non-Volatile: Retains data after power is turned off.
High Capacity: Capable of storing massive amounts of data.
Cost-Effective: Cheaper than SSDs, especially for large storage needs.
Relatively Slow: Slower than RAM or SSDs, with access times measured in milliseconds.
Reliable: Mechanical parts are durable, though susceptible to wear and tear over time.

External HDDs

External HDDs are portable devices used for backing up and transporting data. They complement internal drives by providing additional storage space that can be disconnected and carried as needed.

Hard disk drives remain integral to computing, blending affordability and practicality. Though SSDs are gaining popularity due to their faster speeds, HDDs retain their significance, especially for large-scale storage purposes where reliability, capacity, and cost-effectiveness are priorities.