Pull the plug on a computer and its main memory (RAM) forgets everything in an instant. Yet your photos, your homework, your games and the operating system itself are all still there when you switch back on. Where were they hiding? On secondary storage — the hardware whose whole job is to keep data safely even when the power is off.

Because it holds on to data without electricity, secondary storage is called non‑volatile. This is where files and programs live when they are not being used. When you open an app, the computer copies it from slow, roomy secondary storage into fast, cramped RAM to actually run it; when you save, the changes are written back to secondary storage so they survive the next power‑off.

Almost every device has some — a laptop's internal drive, the USB stick in your pocket, a DVD, a games console's disk, the memory card in a camera. They come in three main families, and this page is about how each one actually works and when you would choose it.

1. Magnetic storage — the hard disk drive (HDD)

A hard disk drive stores data as microscopic magnetised spots on one or more spinning metal platters. A north‑or‑south magnetic direction stands for a 1 or a 0. The platters whirl at thousands of revolutions per minute while a tiny read/write head on the end of a swinging arm floats a hair's breadth above the surface, magnetising spots to write and sensing them to read.

Because the data is arranged in concentric circular tracks, the head has to physically move to the right track and then wait for the spot to spin round underneath it. Those moving parts make HDDs the slowest family and the most easily damaged by a knock — but they are cheap per gigabyte and come in huge capacities (many terabytes), which is why they still fill servers and cheap desktops.

2. Optical storage — CD, DVD and Blu‑ray

Optical discs store data along one enormously long spiral track as a pattern of microscopic bumps and flats — traditionally called pits and lands. A laser shines on the spinning disc: a pit and a land reflect the light differently, and the drive turns that changing reflection into 1s and 0s.

The three types are really the same idea with a shorter‑wavelength laser each generation, so the pits can be packed tighter and the disc holds more:

• CD — infra‑red laser, about 700\text{ MB}.
• DVD — red laser, about 4.7\text{ GB} (single layer).
• Blu‑ray — blue laser, about 25\text{ GB} per layer.

Optical discs are cheap, light and easy to post, which made them perfect for selling music, films and game installers. But they are low capacity by modern standards, slow, and scratch easily — so they are fading fast in the age of downloads and streaming.

3. Solid‑state storage — SSDs and flash

Solid‑state storage has no moving parts at all. Instead of magnetism or lasers, it traps electrical charge inside billions of tiny flash memory cells; the presence or absence of charge stores a 1 or a 0, and the charge stays put with the power off. The same technology appears as a solid‑state drive (SSD) inside a laptop, as a USB flash drive, and as the memory card in a phone or camera.

With nothing to spin or seek, there is no waiting for a head to move — so solid‑state is by far the fastest family, silent, low‑power and shock‑resistant (great for something you carry around). The trade‑off is cost: it is the most expensive per gigabyte, though prices keep falling and SSDs have now replaced HDDs in most new laptops and phones.

Comparing the three families

There is no single "best" secondary storage — each family wins on some measures and loses on others. The usual things to compare are capacity, speed, cost per gigabyte, durability and portability.

Notice the pattern: magnetic gives the most gigabytes for your money, solid‑state gives the most speed and toughness, and optical gives a cheap, mailable disc for distributing a fixed set of files.

Choosing the right type

In an exam you will often be asked to recommend a storage type for a scenario and justify it. The trick is to match the scenario's priority to a family's strength:

• Need speed? (operating system, games, video editing) → SSD, because there are no moving parts to slow it down.
• Need lots of cheap space? (backing up huge video libraries, a file server) → HDD, because it is cheapest per gigabyte.
• Need to carry it or post it? (a file for a friend, distributing a film) → a USB flash drive for read/write portability, or an optical disc for a cheap one‑way handout.
• Need it to survive being dropped? (a device that travels) → solid‑state, because it is shock‑resistant.