What exactly is a hard drive and how does it function?

Almost every desktop computer and server in use today includes one or more hard-disk drives. Every mainframe and supercomputer is typically connected to hundreds of them, and even VCR-like devices and camcorders now utilize hard disks instead of tape. So, what exactly is a hard drive, and how does it manage the countless hard disks?

A hard disk drive (HDD) is a data storage device that employs one or more rigid, rapidly spinning disks coated with magnetic material to read and write data. Hard disks excel at one thing — providing storage for data and operating systems. They also enable computers to retain information during power loss. In this article, we'll disassemble a hard drive to explore its components and discuss how it organizes the gigabytes of information stored in files!

Introduction to Traditional Hard Disk Drives

The invention of the hard disk drive in 1956 was a groundbreaking advancement in data storage technology. Created by IBM engineer Reynold B. Johnson and his team, the first hard disk drive was the IBM Model 350 Disk File, part of the IBM 305 RAMAC computer system. This revolutionary device used magnetic disks for data storage, enabling quick access to vast amounts of data, in contrast to the slower sequential access of tape drives.

The original unit was about the size of two refrigerators and stored just 5 megabytes of data on fifty 24-inch disks. Over the years, HDDs have undergone remarkable transformations in terms of capacity, size, and speed, playing a key role in shaping modern computing and data management.

Hard disks began as large disks, up to 20 inches in diameter, holding only a few megabytes. They were initially called "fixed disks" or "Winchesters" (a codename for a popular IBM product). Later, they were referred to as "hard disks" to differentiate them from "floppy disks." Hard disks have a solid platter that holds the magnetic medium, unlike the flexible plastic film in tape and floppy disks.

At a basic level, a hard disk is quite similar to a cassette tape. Both devices use magnetic recording techniques, as explained in How Tape Recorders Work. Hard disks and cassette tapes also share the key advantages of magnetic storage — the magnetic medium can be easily erased and rewritten, and it retains the magnetic flux patterns stored on it for years.

In this section, we will explore the key differences between cassette tapes and hard drives.

Cassette Tape vs. Hard Disk

Let's examine the major differences between two common storage devices: cassette tapes and hard disks.

Due to these factors, modern hard disks have incredibly high storage capacities and can retrieve data almost instantly.

Storage Capacity and Performance

A typical desktop computer will feature a hard disk with a storage capacity ranging from 10 to 40 gigabytes. Data is stored on the disk as files. A file is essentially a collection of named bytes. These bytes could represent ASCII codes for a text file's characters, instructions for software programs the computer needs to run, database records, or even pixel data for a GIF image.

Regardless of its content, a file is essentially just a sequence of bytes. When a program on the computer needs access to a file, the hard disk fetches its bytes and sends them to the CPU, one byte at a time.

There are two primary metrics used to assess the performance of a hard disk:

Another key factor is the drive's capacity, which refers to the total number of bytes it can store.

Inside: Electronics Board

To truly grasp how a hard disk operates, it helps to take a peek inside. (However, KEEP IN MIND that OPENING A HARD DISK DESTROYS IT, so this is not a task to attempt unless the drive is already nonfunctional.)

This is a typical hard disk drive:

It consists of a sealed aluminum casing, with the controller electronics mounted on one side. These electronics manage the read/write operations and control the motor that spins the disk platters. Additionally, the electronics convert the magnetic domains on the drive into bytes (during reading) and vice versa (during writing). The electronics are housed on a small board that can be detached from the rest of the drive:

Inside: Beneath the Board

Beneath the board, you'll find the connections for the motor that spins the platters, along with a highly filtered vent hole designed to balance internal and external air pressures:

Opening the cover of the drive exposes an interior that is both remarkably simple and incredibly precise:

In this image, you can observe:

Inside: Disk Platters and Heads

To expand the storage capacity, most hard drives feature multiple platters. This particular drive contains three disk platters and six read/write heads:

The mechanism that moves the arms within a hard disk must be extremely fast and precise. A high-speed linear motor is often used for this purpose.

Many hard drives utilize a "voice coil" method, similar to how the cone of a speaker in your stereo system is moved, to control the arm.

Storing Data

Data is stored on a platter's surface in sectors and tracks. Tracks are circular paths, while sectors are pie-shaped segments on each track, like so:

A typical track is marked in yellow, while a sector is shown in blue. Each sector holds a specific number of bytes—commonly 256 or 512. Sectors are often grouped into clusters either at the drive or operating system level.

Low-level formatting a drive sets up the tracks and sectors on the platter, marking the beginning and end of each sector. This step prepares the drive to store blocks of bytes. High-level formatting then writes the file-storage structures, such as the file-allocation table, into these sectors, preparing the drive to store files.