Unlike primary storage (RAM), secondary storage cannot be directly accessed by the CPU. However, secondary storage is non-volatile, unlike RAM, meaning it retains data when it is turned off. Therefore, RAM is used for programs and data currently being used while secondary storage is used for long-term storage (files, applications, OS etc.). Secondary storage is also cheaper per gigabyte and far larger than primary storage.
On this page, we will first cover what you need to know for the A-Level, and then go into far more depth in how the different types of storage actually work.
A-Level content
On the A-level specification, there isn't actually a lot you need to know. The subject content clarification says:
"Candidates need to understand that there are different types of storage device. They need to know about the characteristics of each type (magnetic, optical and flash) and understand the benefits and drawbacks of each, and be able to recommend an appropriate type of device for a given situation and justify the choice."
Here is a table summarising everything you need to know for A-Level:
Magnetic
Optical
Flash
Characteristics
Data stored on spinning disks
Uses read/write heads
Has moving parts
Data stored in lands and pits
Read with a laser
Typically removable media
Data stored in transistors with electrons
No moving parts
Benefits
Large capacity
Cheapest per GB
Reliable for long-term storage
Portable
Cheap to produce
Good for distribution
Very fast read/write speeds
Durable
Low power consumption
Portable
Drawbacks
Slower access speeds
Fragile (moving parts)
Not portable
Limited capacity
Very slow read/write speeds
Easily scratched
More expensive per GB
Limited write cycles
Typical uses
PCs and servers
Backups and archives
Distribution of media e.g. music
Backup and archiving
Laptops and PCs
File transfer
SD cards in phones and cameras
Exam practice
William is creating a film for a school project using a digital video camera. The digital video camera has a secondary storage device.
(a) Explain why the digital video camera needs secondary storage. [2]
For long term/permanent/non-volatile storage
storing when the device is turned off
To store the videos/data/files
For transferring the videos (to another device)
The digital video camera uses solid state storage.
(b) Explain why solid state storage is the most appropriate type of storage for the digital video camera. [4]
Portable
Lightweight
…e.g. device needs to be carried
Small physical size
Way more points which I can't be asked to add
Beyond the A-Level curriclum
Now that we've covered all of the A-level content and had some exam practice on it, we can get onto some fun extension stuff!! In this section, we will learn in detail how the different types of secondary storage actually work.
Optical storage
Optical storage devices like CDs, DVDs, and Blu-ray discs work by using a laser to read data from and write data to a disc's surface, which is encoded with tiny physical bumps called pits and flat areas called lands.
Reading data involves a low-power laser reflecting differently off pits and lands, which is then detected by a sensor and translated into binary code (0s and 1s).
Magnetic storage (HDD)
Magnetic storage devices work by using magnetic fields to magnetize tiny sections on a surface, representing data as binary 1s and 0s.
A read/write head moves across a spinning disk (like in a hard drive) or a magnetic tape to magnetize or demagnetize these sections to write data, and then detects the magnetic orientation to read the data back.
In a hard disk drive, the data is stored in concentric circles called tracks on spinning platters. In magnetic tape, the data is arranged in parallel tracks along the length of the tape.
Flash storage (SSD)
Flash storage works by trapping electrons in tiny cells to represent data as binary ones and zeros.
A flash memory cell contains a floating-gate transistor, which can store a charge even when power is off (making it non-volatile).
When a high voltage is applied, electrons tunnel through an insulating layer and get stuck in the floating gate, representing a binary 0. A controller reads the data by checking if electrons are trapped in the cell.
