A storage device (also called secondary storage or backing storage) keeps programs and data permanently, even when the power is off. Your files, your apps and the operating system itself all live on storage until they are needed, and then they are copied into RAM so the CPU can work on them. We compare storage devices using four features: capacity, access speed, access method and portability.

This lesson follows on from Memory Types and Features. If you did CSEC, Inside the Computer: Input, Processing, Memory, Storage and Output (IPOS) is a short refresher.

Storage and memory are not the same

The four features

Capacity

Capacity is how much data the device can hold, measured in bytes: MB, GB and TB. Remember 1 KB = 1024 bytes, 1 MB = 1024 KB, 1 GB = 1024 MB and 1 TB = 1024 GB. (Drive makers often use 1000 instead of 1024, which is why a new “1 TB” drive shows a little less space on a computer.)

Access speed

Access speed is how quickly data can be found and read or written. Two measures are used:

• Access time: how long it takes to find the data. On a hard disk this is the seek time (moving the head to the right track) plus the rotational delay (waiting for the right sector to spin under the head).
• Data transfer rate: how much data can be moved per second once it has been found, e.g. 150 MB/s.

Devices with moving parts (tape, hard disk, optical disc) are slower to access than solid state devices, which find data electronically.

Access method

The access method is the way the device gets to a particular piece of data:

• Sequential (serial) access: the data is read in order from the start, so to reach a record, every record before it must be passed. Magnetic tape works this way.
• Direct (random) access: the device can go straight to the data using its address. Disks (magnetic and optical) and solid state devices work this way. A hard disk is sometimes called semi-random, because the head goes straight to the track but then waits for the sector to come round.

More on this in Serial, Sequential, Direct (Random) Access and File Organisation and Access Methods.

Portability

Portability is how easily the device (and its data) can be carried and used on another computer. Small size, low weight, no separate power supply, a common connector such as USB, and the ability to survive knocks all make a device more portable. A USB flash drive is very portable; an internal hard disk is not meant to be moved.

Types of storage devices

Magnetic storage: hard disk and tape

A hard disk drive (HDD) stores data magnetically on spinning metal or glass platters. Each platter surface is divided into circular tracks, and each track into sectors. A read/write head on an actuator arm moves across the platter to the right track. Hard disks give a large capacity at a low cost per GB and allow direct access, but because of the moving parts they are slower than SSDs, use more power and can be damaged if dropped while running.

Image: Hard drive.svg by Surachit, CC BY-SA 3.0, via Wikimedia Commons.

Magnetic tape stores data on a long plastic tape coated with magnetic material, wound inside a cartridge. Access is sequential, so finding one file can take a long time. But a tape cartridge holds a huge amount of data (18 TB or more) very cheaply and lasts for years, so tape is used by banks, businesses and data centres for backups and archives, where the data is written and read in order anyway.

Optical storage: CD, DVD and Blu-ray

Optical discs store data as tiny pits and flat areas (lands) along one long spiral track. A laser shines on the track and the drive detects how the light is reflected. Blu-ray uses a blue-violet laser with a shorter wavelength than the red laser of a DVD, so the pits can be smaller and packed closer together, giving more capacity.

• CD: about 700 MB. DVD: 4.7 GB (8.5 GB dual layer). Blu-ray: 25 GB per layer (50 GB dual layer).
• -ROM discs are pressed at the factory and are read only; -R discs can be written once; -RW discs can be erased and rewritten.
• Optical discs allow direct access but are slow, and they are cheap, light and very portable. They are used for distributing films, music and software, and for small backups, but they are much less common now than flash drives and the cloud.

Solid state storage: SSD, flash drive and memory card

Solid state devices store data in flash memory chips, a type of EEPROM (see Memory Types and Features). There are no moving parts, so they find data very quickly, use little power, are silent and survive knocks better than hard disks. They cost more per GB than hard disks, and each cell can only be rewritten a limited number of times.

• Solid state drive (SSD): replaces the hard disk inside most laptops and many desktops, so the computer starts and opens programs much faster.
• USB flash drive: pocket-sized, plugs into a USB port, ideal for carrying files between computers.
• Memory card (SD, microSD): tiny, used in cameras, phones, drones and game consoles.

Cloud storage

Cloud storage keeps your files on a provider’s servers (for example Google Drive or OneDrive), which you reach over the Internet. The real storage devices are hard disks and SSDs in the provider’s data centres. Capacity depends on your plan, there is nothing to carry, and your files are backed up off site. But you need an Internet connection, the speed depends on that connection, and you are trusting the provider with your data. See Cloud Storage vs Local Storage.

Capacity and copying time

To check whether data fits, compare its size with the capacity, using the same unit. To estimate the copying time, divide the size by the transfer rate:

time (seconds) = size (MB) ÷ transfer rate (MB/s)

For example, a 4 GB video is 4 × 1024 = 4096 MB. At 100 MB/s it takes 4096 ÷ 100 ≈ 41 seconds. Watch the units: Internet speeds are given in megabits per second (Mbps), and there are 8 bits in a byte, so a 10 Mbps upload is only 10 ÷ 8 = 1.25 MB/s.

Files live on storage: a C example

In Memory Types and Features we saw that a variable lives in RAM and is lost when the program ends. To keep a value, a program must write it to a file on a storage device. The first program below saves the value of score to a file with fopen and fwrite. The second program, run later (even after a restart), reads it back with fread.

#include <stdio.h>

int main()
{
    int score = 95;
    FILE *f = fopen("score.dat", "wb");

    if (f == NULL)
    {
        printf("Could not open file\n");
        return 1;
    }

    fwrite(&score, sizeof(score), 1, f);
    fclose(f);
    printf("Saved %d\n", score);

    return 0;
}

#include <stdio.h>

int main()
{
    int score = 0;
    FILE *f = fopen("score.dat", "rb");

    if (f == NULL)
    {
        printf("Could not open file\n");
        return 1;
    }

    fread(&score, sizeof(score), 1, f);
    fclose(f);
    printf("Read %d\n", score);

    return 0;
}

"wb" means write in binary mode and "rb" means read in binary mode. fwrite copies the bytes of score from RAM to the file, and fread copies them from the file back into RAM. Notice that the second program starts with score = 0, so any other value it prints must have come from storage. If you would rather store data as readable text, see Tutorial: Storing and loading data in a plain textfile.

Practice questions

Homework

1. Copy the comparison table from Try it 2 into your notebook. Add a column "One suitable use" and fill it in for each device.
2. Find the storage on a computer or phone at home: the type (HDD, SSD or flash), the capacity and how much is free. Write down where you found the information.

Exam tips

• Specimen Unit 1 Paper 01 (2021), Item 8. A multiple-choice item on comparing how quickly different memory and storage devices give access to data (syllabus reference 1.1, Module 1). It tests whether you can rank electronic memory against storage devices, including older removable media you may not have used yourself. Put the options in order of speed in your head (registers, cache, RAM, then storage, with moving parts slowest) before you choose. Open the specimen paper and try it before you check the answer key.
• 2024 Unit 1 Paper 02, Question 1(b). Two parts that compare a storage device with RAM: a hard disk against RAM on access method and access speed, and RAM against a flash drive on volatility and portability, 3 marks each. It tests whether you can use the storage features from this lesson to explain a difference. For each named feature, say something about both devices and give a reason, for example the head has to move to the track and wait for the sector, so hard disk access is measured in milliseconds. Try it first, then compare with the solution on this site: 2024 U1 Q1.
• 2017 and 2018 Unit 2 Paper 02, Question 5. Storage comes back in Unit 2 with virtual memory, where pages of processes that do not fit in RAM are kept on the hard disk. Try the questions, then see the solutions on this site: 2017 U2 Q5 and 2018 U2 Q5.
• More worked answers are on the CS Solutions page.

General advice for storage questions (from the lesson content, not from a particular paper): when asked to choose a device, name it and justify it using the four features, capacity, access speed, access method and portability, linked to the scenario.

Previous: Memory Types and Features. Next: the processor (CPU) and how it fetches instructions and data from memory (coming soon in this course; for now see 2009 U1 Q2 Part C). After that, data representation: how the bits on these devices stand for numbers and characters (Decimal to binary, Binary Manipulation).

Syllabus link: CAPE Computer Science (CXC A19/U2/21), Unit 1, Module 1: Computer Architecture. Specific Objective 1: state the purpose of the main components of a computer system (storage devices: capacity, access speed, access method, portability). U1 M1 SO1.

Attributions to media used in this post:

Hard disk diagram: Surachit, CC BY-SA 3.0, via Wikimedia Commons (File:Hard drive.svg).

Updates

7 Nov 2026: Added the shared course badge summary and lesson mastery quiz, with feedback, retries and browser-saved progress.

27 Oct 2026

- Draft created.

28 Oct 2026

- Exam references reworded and quiz questions rewritten.

© 2026 Vedesh Kungebeharry. All rights reserved.