Key topics for OCR GCSE Computer Science:  Secondary Storage Devices

For the OCR Exam you need to know:

Why do we need secondary storage?

How to calculate data capacity?

What are the 3 common types of storage devices and how do they work?

What are the advantages and disadvantages of different storage devices when considering:

  • Capacity
  • Speed
  • Cost
  • Durability
  • reliability
  • Portability

Primary Storage

Primary storage is the CPU’s memory. Due to it’s volatile nature it is very very fast.

It consists of:

  • Cache
  • Ram

Whilst primary storage is fast, it has 2 problems:

  • It is quite small ( RAM 8GB, Cache 512Kb)
  • It is volatile – when you turn off the computer, that data goes!

Because of this we need secondary storage!

Cache memory is located in the CPU]
Random Access Memory – A cheap and easy component to upgrade on your pc

Secondary Storage

Secondary storage is:

  • Non – volatile – the data remains even when the power is gone.
  • Slower than primary storage
  • Much larger in capacity

Examples of secondary storage:

  • HDD
  • CD Rom Drives
  • ROM Chips
  • USB Sticks
  • SSD Drives
  • Magnetic tape drives

Revision Sheets




Magnetic Storage Devices

These are the oldest of the commonly used storage devices, but are still the most widely used.

  • Magnetic Hard Drives are used to store data and programs on desktops and laptops
  • Magnetic tape drives are usually used for large server / systems backups.


  • Cheap Storage per MB (especially tape drives!)
  • Relatively quick read / write speeds.


  • Easily broken if dropped
  • Slow read /write compared to new SSD drives.
Magnetic HDD With spinning platter in view
Magnetic tape drives

How they work

Also take a look at this hard drive YouTube video by the slo-mo guys, awesome!

Optical Storage Devices

Types of optical storage devices:

  • CD Drives
  • DVD Drives
  • BluRay Drives

Optical storage devices offer cheap and portable high capacity secondary storage. Far more portable than an internal harddrive, which makes them good for small / medium size backups and great for sending through the post.

How they work

Optical storage devices work by firing a laser at the surface of a spinning disk. The disc is covered in a pattern of pits in the CD surface. As the laser hits the pits it is reflected and the pattern of pits it detected by a laser detector.

Solid State Storage Devices

Solid state storage devices work by storing data in flash chips electronically.


  • Very fast read / write speeds.
  • Shock resistant as there are no moving parts to break if knocked


  • More expensive per GB of storage
  • Limited life span, as each flash cell can only be written to a limited number of times.
SSD Drive – Expensive but fast

Very portable and small – but needs a reader to use