How is a 1 or a 0 stored on a hard disk drive?

As areas magnetised in one of two directions

How is data read from a DVD?

A laser detects differences in reflection from pits and lands

Which is the main reason solid state drives are used in laptops?

They have no moving parts, so they are fast, robust and use little power

How secondary storage works

Your files must survive when the power goes off. Secondary storage does this in three very different ways: magnetism, light and trapped electrical charge.

GCSE · 1CP2 · 3.1.2
International GCSE · 4CP0 · 4.2.5
About 15 minutes

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Why secondary storage?

Main memory (RAM) is volatile: it loses its contents when the power is turned off. Secondary storage is non-volatile, so it keeps the operating system, programs and files for later.
It is also much larger and cheaper per gigabyte than RAM, but slower, so programs are copied into RAM to run.

Magnetic storage

A hard disk drive (HDD) has metal platters coated in magnetic material that spin at high speed. A read/write head on a moving arm floats just above each surface.
Tiny areas of the surface are magnetised in one of two directions, which represent 0 and 1. Writing changes the magnetisation; reading detects it.
Magnetic tape works the same way and is still used for large backups and archives.
Strengths: very large capacity and low cost per gigabyte. Weaknesses: moving parts make it slower, noisier and easier to damage if knocked.

Optical storage

CDs, DVDs and Blu-ray discs store data on a spiral track. A laser shines on the spinning disc. The surface has pits and flat areas called lands, which reflect the laser light differently. A sensor detects these changes and they are read as binary.
Recordable discs are written by a stronger laser that changes the surface.
Strengths: cheap, light and easy to post. Weaknesses: small capacity, slow, and easily scratched.

Solid state storage

Solid state drives (SSDs), USB flash drives and memory cards use flash memory. Each memory cell traps (or does not trap) electrical charge, and this represents a 1 or a 0. There are no moving parts.
Strengths: very fast access, robust, silent, light and low power, which suits laptops and phones. Weaknesses: more expensive per gigabyte than a hard disk, and each cell can only be rewritten a limited number of times.

	Magnetic (HDD)	Optical	Solid state (SSD)
How bits are stored	Magnetised areas	Pits and lands read by a laser	Trapped electrical charge
Moving parts?	Yes	Yes (spinning disc)	No
Speed	Medium	Slow	Fast
Cost per gigabyte	Low	Low	Higher
Robustness	Damaged by knocks	Scratches easily	Very robust

Worked example

A film studio must keep 40 TB of video for many years and wants the lowest cost. A camera operator needs fast storage that survives being dropped. Recommend storage for each.

The studio needs huge capacity at low cost per gigabyte, and speed matters less: magnetic hard disks (or tape).
The camera needs speed and robustness, so no moving parts: solid state storage such as a memory card or SSD.

Answer: Studio: magnetic storage. Camera: solid state storage.

Key idea

Secondary storage is non-volatile. Magnetic storage uses magnetised areas on spinning platters; optical uses a laser to read pits and lands; solid state traps electrical charge in flash memory and has no moving parts.

Key words

Secondary storage: Non-volatile storage for files and programs: magnetic, optical or solid state.
RAM: Volatile main memory holding the programs and data in use.
Volatile: Loses its contents when the power is off, like RAM.
Operating system: System software that manages files, processes, peripherals, users and the user interface.
Binary: Base 2: the number system that uses only the digits 0 and 1.

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Quick check

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Extra practice

2 more questions

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How is a bit stored on a CD or DVD?
1. As areas magnetised in one of two directions
2. As trapped electrical charge
3. As pits and lands on the surface
4. As holes punched in the disc
Show the answer
Answer: As pits and lands on the surface
Optical discs use pits and lands.
How is data read from a magnetic hard disk?
1. A laser reads pits and lands
2. A read/write head detects the direction of magnetisation as the platter spins
3. A camera photographs the platter
4. Charge flows through cells
Show the answer
Answer: A read/write head detects the direction of magnetisation as the platter spins
Hard disks are read magnetically.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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