How do external hard drives store data?

External hard drives are extremely useful devices that provide additional storage space for computers, servers, smartphones, gaming consoles, and other electronics. They allow you to store large amounts of data externally from your main devices. But how exactly do they store all that data? Let’s take a closer look at the internal components and data storage methods used by external hard drives.

What are the key components inside an external hard drive?

An external hard drive has all of the same key components found inside an internal hard drive. These include:

  • Magnetic platters – These flat, circular discs are made of metal or glass and are coated on both sides with a thin magnetic material. The platters rotate at very high speeds inside the hard drive enclosure. The presence or absence of magnetism in specific areas on the platters is used to store data.
  • Read/write heads – These tiny electromagnetic devices float just above the surface of each platter, reading and writing data as the platters spin. One head is used to access each side of every platter.
  • Spindle – This is a rod that rotates the platters at high speed. In most modern hard drives, the spindle is powered by a brushless DC motor.
  • Actuator arm – The actuator arm holds the read/write heads in place and uses an electro-magnetic coil to move the heads back and forth across the platters as needed to access data.
  • Firmware – This is low-level software that provides instructions for controlling the electromechanical components of the drive and interfacing with the computer.
  • Interface – This is the hardware and protocols that allow communication between the drive and computer, such as USB, FireWire, eSATA, etc.
  • Casing – The metal or plastic casing encloses and protects all of the sensitive internal components.

In addition, external hard drives need a:

  • Power adapter – This converts AC power from a wall outlet to the DC power needed by the drive electronics.
  • External interface – This is the hardware that bridges the communication between the USB/FireWire/eSATA port on the enclosure and the SATA or IDE interface used internally.

How is data written to the platters?

Data is written to the platters using magnetic polarities. Each platter is magnetically coated with a large number of microscopic areas called magnetic domains, each of which can have either a north or south magnetic alignment.

To write data, the read/write heads apply a magnetic field to specific domains, aligning them in a pattern that encodes the binary 1s and 0s of digital data. The presence of a magnetic domain aligned in a north or south direction represents a 1 or 0.

For example, this microscopic pattern of magnetic domains translates to the binary data “01001010”:


The platters retain this magnetic alignment even when the drive is powered down. So the data is safely persisted.

How is data read from the platters?

When reading data, the read heads detect the magnetic orientation of the domains as they pass under them. The north or south alignment of each domain generates a tiny positive or negative electric pulse in the head.

These pulses are then decoded back into the original 1s and 0s of the binary data. This happens extremely rapidly, enabling data to be read from the drive and transmitted to the computer.

How is data located on the drive?

Data organization on hard drives can be visualized as concentric circles called tracks on each platter surface, divided into small arcs called sectors.

Hard drive diagram

The firmware knows the exact track and sector location of every piece of data on the drive. When data is requested by the operating system, the firmware directs the actuator arm to move the heads to the correct track and then waits for the target sector to pass under the read/write head. This location-based system allows data to be efficiently located, written and read.

On most consumer hard drives, each track is divided into several hundred sectors. The sectors near the rim of the platter have greater length than those closer to the spindle. This optimizes storage density by keeping the data bits closer together.

How is data organized into files?

The sectors on the platters contain only the raw 1s and 0s of data storage. The OS manages file storage in a separate layer called the file system. This tracks the locations of files on the drive and maintains directory structures and filenames.

Two key file system structures are:

  • File allocation table (FAT) – Tracks which clusters of sectors belong to each file.
  • Directory table – Maintains the folder hierarchy and filenames.

The file allocation table indicates which sectors belong to which file clusters. Knowing the starting sector of the first cluster, the OS can then follow the FAT chain to reconstruct entire files when needed. The directory table contains file names and maps them to the first cluster allocated for each file.

Some common file systems used by external hard drives include FAT32, exFAT, NTFS, HFS+, and EXT4. The choice of file system depends on the main operating system the drive will be used with.

Why does HDD capacity double approximately every 2 years?

Hard drive storage capacity has grown at a remarkable pace over the decades, roughly doubling every 2 years – a rate faster than Moore’s Law for processors. There are several factors behind this rapid growth:

  • Higher areal density – This refers to the number of bits that can be stored per square inch on the platter surfaces. Smaller magnetic domains and tracks allow for greater density.
  • More platters – Increasing the number of platters to 3, 4 or 5 within a standard drive height boosts capacity.
  • Shingled Magnetic Recording (SMR) – This newer technique overlaps tracks in a shingle-like pattern to squeeze more tracks onto each platter.
  • Helium drives – Filling drives with helium instead of air allows thinner platters and closer magnetic spacing while still avoiding crashes.
  • Stronger error correction codes (ECC) – More advanced ECC improves accuracy and allows smaller magnetic domains.

Rapid innovations in all of these areas have enabled hard drive vendors to steadily grow the storage capacities of HDDs year after year.

External HDD interface speed comparison

The speed of data transfer to and from external hard drives depends largely on the interfaces they use to connect to PCs and other devices. Here is a comparison of common external hard drive interface speeds:

Interface Speed
USB 2.0 480 Mb/s (60 MB/s)
USB 3.0/3.1 Gen 1 5 Gb/s (625 MB/s)
USB 3.1 Gen 2 10 Gb/s (1250 MB/s)
USB 3.2 Gen 2×2 20 Gb/s (2500 MB/s)
USB4 40 Gb/s (5000 MB/s)
eSATA 6 Gb/s (750 MB/s)
FireWire 400 400 Mb/s (50 MB/s)
FireWire 800 800 Mb/s (100 MB/s)
Thunderbolt 1 10 Gb/s (1250 MB/s)
Thunderbolt 2 20 Gb/s (2500 MB/s)
Thunderbolt 3 40 Gb/s (5000 MB/s)

As you can see, both USB and Thunderbolt interfaces have continued to evolve, with each new version offering significantly higher maximum bandwidths. This allows fast external storage solutions to take advantage of the speeds enabled by SATA and NVMe internal drives.

Do SSDs work differently than HDDs?

Solid state drives (SSDs) differ quite a bit from hard disk drives in how they store data. Instead of magnetic platters, read/write heads and moving parts, SSDs utilize interconnected flash memory chips to store data.

Here are some key differences in how SSDs work:

  • No moving parts – SSDs are purely electronic with no mechanical components or spinning platters.
  • NAND flash memory – SSDs store data in an array of NAND flash chips made up of millions of tiny memory cells.
  • Electrical pulses – Instead of magnetism, SSDs program memory cells by trapping electrons inside them, with different voltage levels representing 1s and 0s.
  • Page and block organization – Data is organized into pages (4-16 KB) and blocks (256+ pages) across the flash chips.
  • Wear leveling – To extend longevity, SSD controllers spread writes across all cells evenly.
  • File system support – Like HDDs, SSDs rely on the OS file system for logical organization of files.
  • No seek time – SSDs have near instant data access without seek time delays.

Due to these differences, SSDs provide much faster access, higher durability and shock resistance compared to traditional hard disk drives. However, HDDs continue to offer larger capacities at lower prices per gigabyte.


External hard drives continue to be a popular, affordable way to add significant storage space to desktop and laptop computers. The evolution of platter, head and magnetic recording technologies have steadily pushed hard drive capacities higher while costs have dropped dramatically.

Interface speeds have also improved greatly, with modern connections like USB 3.2, Thunderbolt 3 and USB4 supporting theoretical maximum throughputs measured in gigabytes per second. Solid state drives provide an even faster external storage option, but HDDs still excel in terms of cost per terabyte. Carefully considering interface speed, capacity, physical size and cost requirements will help you select the best external storage solution for your needs.