What you'll learn
Storage devices keep data and programs when the power is off. This guide explains the difference between primary memory and secondary storage, the three main storage technologies — magnetic, optical and solid state — and cloud storage. You will learn to compare devices by capacity, speed, portability, durability and cost, to choose a suitable medium for a task, and to explain good backup practice. It also covers units of storage, file sizes and compression, because questions often ask how much a device can hold.
Most marks in this topic come from choosing and justifying a storage method for a scenario. A good answer always names a specific benefit linked to the user's needs — "an SSD has no moving parts, so it is less likely to be damaged when the engineer's laptop is knocked in the van" — rather than a general statement.
Key terms and definitions
Primary storage (memory) — Storage the CPU can access directly: RAM, ROM and cache.
Secondary storage — Non-volatile storage used to keep data and programs long term, such as hard disks, SSDs and optical discs.
Volatile — Loses its contents when the power is switched off.
Non-volatile — Keeps its contents without power.
Magnetic storage — Stores data as magnetised areas on a surface, such as hard disk drives and magnetic tape.
Optical storage — Stores data as pits and lands read by a laser, such as CDs, DVDs and Blu-ray discs.
Solid state storage — Stores data in flash memory chips with no moving parts, such as SSDs, USB flash drives and memory cards.
Cloud storage — Storing files on remote servers accessed over the internet.
Capacity — How much data a device can hold.
Access speed — How quickly data can be read or written.
Backup — A copy of data kept so that it can be restored if the original is lost or damaged.
Serial access — Data must be read in order from the start, as on magnetic tape.
Direct (random) access — Data can be read from any location immediately, as on a disk.
Core concepts
Why secondary storage is needed
RAM is fast but volatile and relatively expensive, so it cannot keep files when the power is off and is too small to hold everything. Secondary storage is non-volatile and much cheaper per gigabyte, so it holds the operating system, applications and users' files permanently. When a program runs, it is copied from secondary storage into RAM. A computer needs both: RAM for speed while working, storage for keeping things.
Magnetic storage
Hard disk drives (HDDs) contain spinning platters coated with magnetic material, with read/write heads that move across them. They offer high capacity at low cost per gigabyte, which makes them good for desktops, servers and bulk storage. However, the moving parts make them slower than SSDs, noisier, more power-hungry and vulnerable to damage if dropped while running.
Because files on a hard disk can become split into pieces stored in different places, defragmentation can speed up access by rearranging the pieces so they are stored together. It is not useful on SSDs, where access time does not depend on physical location, and the extra writing would cause unnecessary wear.
Magnetic tape has very high capacity and very low cost per gigabyte, and it lasts well when stored properly. It uses serial access, so finding one particular file can take a long time. That makes it unsuitable for everyday use but ideal for archiving and large-scale backups that are rarely needed.
Optical storage
CDs, DVDs and Blu-ray discs are read by a laser. Capacities rise from CD (around 700 MB) to DVD (around 4.7 GB single layer) to Blu-ray (25 GB single layer and more). Discs are cheap, light and easy to post, but they scratch, have limited capacity and are slow compared with other media.
There are three types:
- ROM (read-only) — data written at manufacture and cannot be changed, as with films and software sold on disc.
- R (recordable) — can be written once, then read many times.
- RW (rewritable) — can be written, erased and rewritten many times.
Solid state storage
Solid state drives (SSDs), USB flash drives and memory cards store data in flash memory chips. With no moving parts, they are:
- much faster than hard disks, so computers start and load programs quickly;
- more durable when knocked or dropped;
- silent and low in power use, which helps laptop battery life;
- small and light.
They cost more per gigabyte than hard disks, and each memory cell can only be rewritten a large but limited number of times. USB flash drives are ideal for carrying files between places; memory cards are used in cameras and phones.
Cloud storage
Cloud storage keeps files on servers owned by a provider and accessed over the internet.
Advantages: files can be accessed from any device anywhere; storage can be increased easily; the provider handles hardware and often keeps its own backups; files are protected if a local device is lost or damaged; sharing and collaboration are easy.
Disadvantages: an internet connection is needed to access files; large uploads and downloads can be slow; there are ongoing subscription costs; the user depends on the provider's security, reliability and continued existence; personal or sensitive data may be stored in another country, raising legal and privacy questions.
Comparing storage
| Medium | Capacity | Speed | Portability | Durability | Cost per GB |
|---|---|---|---|---|---|
| HDD | High | Medium | Low–medium | Damaged if dropped | Low |
| SSD | Medium–high | Very high | High | High | Higher |
| USB flash drive | Low–medium | Medium | Very high | High | Medium |
| Optical disc | Low | Low | High | Scratches | Low |
| Magnetic tape | Very high | Low (serial) | Medium | Good if stored well | Very low |
| Cloud | Scalable | Depends on connection | Accessible anywhere | Provider-managed | Subscription |
Units and file sizes
Storage is measured in bytes: 1 KB = 1,024 bytes, 1 MB = 1,024 KB, 1 GB = 1,024 MB, 1 TB = 1,024 GB. Different file types need very different amounts of space. Plain text is tiny; photos are larger; video is largest because it stores many images per second plus sound, and higher resolutions such as 1080p store more pixels per frame than 720p. Devices often show less usable space than advertised — partly because manufacturers count 1 GB as 1,000,000,000 bytes while operating systems count in units of 1,024, and partly because some space is used for formatting and system files.
Compression reduces file size so that more fits in storage and files send faster. Lossy compression removes data permanently, so it should never be used on the only copy of an archive photo you want to keep at full quality.
Backups
A backup is a copy kept so data can be restored after loss from hardware failure, theft, fire, accidental deletion or malware. Good practice:
- back up regularly, according to how often data changes;
- keep at least one copy in a different location from the original, so a fire or theft does not destroy both;
- keep at least one copy offline or separate from the network, so ransomware cannot encrypt it;
- test that backups can actually be restored;
- protect backups with the same security as the original data, as they contain the same information.
Organisations often combine methods, such as a local external drive for quick restores and cloud storage for an off-site copy. A school server might also mirror its disks, writing identical data to two drives so that if one fails the other carries on.
Worked examples
Example 1: Choosing storage for a field engineer
A field engineer uses a laptop in a van and on building sites. Explain why an SSD is more suitable than a hard disk. (2 marks)
Answer: An SSD has no moving parts, so it is much less likely to be damaged if the laptop is knocked or dropped on site (1). It also uses less power, so the battery lasts longer away from mains electricity (1).
Example 2: Long-term archiving
A company must keep seven years of records that will rarely be accessed. Which medium is most suitable?
Answer: Magnetic tape. It has very high capacity at very low cost per gigabyte and keeps data reliably for long periods. Its slow serial access does not matter because the records are rarely needed.
Example 3: Calculating capacity
How many 4 MB photos can be stored on a 16 GB memory card? (Use 1 GB = 1,024 MB.)
16 GB = 16 × 1,024 = 16,384 MB. 16,384 ÷ 4 = 4,096 photos (in practice slightly fewer, as some space is used by the system).
Example 4: Evaluating cloud backup
A small business currently backs up to an external hard drive kept in the office. It is considering cloud backup. Evaluate the change.
Answer: Cloud backup stores a copy off-site, so a fire or burglary at the office would not destroy both the data and the backup, and backups can happen automatically. However, the business would depend on its internet connection to restore data, pay ongoing fees, and need to trust the provider with sensitive records. Keeping both — the local drive for quick restores and the cloud as an off-site copy — gives the best protection.
Common mistakes and how to avoid them
- Calling RAM "storage". RAM is primary memory; hard disks and SSDs are secondary storage.
- Writing "SSDs are faster" without explaining why. Add "because there are no moving parts to wait for".
- Recommending tape for everyday files. Its serial access makes it slow to find individual files.
- Keeping backups next to the original. Location matters; off-site and offline copies protect against fire, theft and ransomware.
- Ignoring the scenario. Portability, durability, capacity and cost matter differently for a photographer, a school and a bank.
Exam technique for "Storage Devices and Media"
- Learn the comparison criteria — capacity, speed, portability, durability, reliability, cost — and use them as the backbone of every comparison answer.
- For "choose and justify" questions, name the device, then give two reasons linked directly to the scenario.
- Show working for capacity calculations and state whether you used 1,000 or 1,024.
- In cloud questions, always give both sides: anywhere access and off-site safety versus dependence on internet, cost and trust in the provider.
Quick revision summary
- Secondary storage is non-volatile and keeps data when the power is off.
- Magnetic: HDD (cheap, high capacity, moving parts), tape (huge, cheap, serial — archives).
- Optical: CD, DVD, Blu-ray; ROM, R and RW versions; cheap but low capacity and easily scratched.
- Solid state: SSD, USB, memory cards — fast, durable, silent, low power, more expensive per GB.
- Cloud: access anywhere and off-site safety, but needs internet, costs money and relies on the provider.
- Back up regularly, off-site, with an offline copy, and test restores.