3.7 Secondary Storage Trainer
Magnetic, optical and solid state — how each works, their advantages, and when to use each. Cambridge's favourite comparison topic.
3.7 Path
Mistakes
Stats
Bookmarks
Book Notes
Knowledge Vault
Add entry
Entries
What do I need to know before I start?
Secondary storage holds data permanently — it isn't accessed directly by the CPU, and it keeps its contents when the power is off. There are three types: magnetic (HDD), optical (CD/DVD/Blu-ray) and solid-state (SSD, USB flash, SD card). The exam wants you to know how each physically works, name real examples, and — critically — get the exact terminology right. Several confirmed examiner traps live in this topic, so precise wording matters more than usual.
By the end of 3.7 you can…
| Objective | Why it matters |
|---|---|
| State the purpose of secondary storage precisely | s24 examiner report: it's "to store data permanently until deleted by the user" — NOT "to make a backup". |
| Name the three types and give real examples | Magnetic (HDD), optical (CD/DVD/Blu-ray), solid-state (SSD, USB flash drive, SD card). |
| Describe how each type physically stores data | Magnetic dots, laser-burned pits, or electrical charges in transistors. |
| Know that an HDD is magnetic, not optical | s24 examiner report: the most common mistake — HDDs spin like optical discs but use an electromagnet, not a laser. |
| Compare solid-state and optical correctly | s24 examiner report: "faster" alone scores 0 — say faster to access data. |
| Use full device names | s22 examiner report: "USB" alone scores 0 — write "USB flash memory drive" / "USB memory stick". |
Cambridge-approved terms
Non-volatile storage that holds data permanently until deleted. Not directly accessed by the CPU.
Spinning platters; a read/write head uses an electromagnet to store data as magnetised dots. Example: HDD.
A laser burns pits onto a reflective disc and reads the reflected light. Examples: CD, DVD, Blu-ray.
No moving parts; data stored as electrical charges in transistors (NAND flash). Examples: SSD, USB flash drive, SD card.
Hard Disk Drive — the common example of magnetic secondary storage.
Solid-State Drive — solid-state storage; faster to access than an HDD, with no moving parts.
The three types of secondary storage
| Type | How it works | Examples |
|---|---|---|
| Magnetic | Platters spin; a read/write head uses an electromagnet; data stored as magnetised dots, organised in tracks and sectors. | HDD |
| Optical | A laser burns pits onto a reflective disc surface; the laser reads light reflected from pits and lands along a spiral track. | CD, DVD, Blu-ray |
| Solid-state | No moving parts; transistors in a grid on semiconductor chips (control + floating gates); data stored as electrical charges (NAND). | SSD, USB flash drive, SD card |
Solid-state vs optical
Solid-state is faster to access data and has no moving parts, so it's more durable; optical media are cheap and good for distributing files, but slower and easily scratched. Always say faster/durable at what — never just "better".
Traps that cost marks every session
s24 examiner report — the most common error. An HDD is magnetic: spinning platters read by an electromagnet, not a laser.
s24 examiner report: the purpose is "to store data permanently until deleted by the user". Backup is a use, not the purpose.
s22 examiner report: "USB" scores 0 — it's a connector. Write "USB flash memory drive" or "USB memory stick".
s24 examiner report: say faster at what — "solid-state is faster to access data".
Only primary storage is directly accessed by the CPU. Secondary storage isn't.
An SSD is solid-state — no moving parts, data stored as electrical charges. Only the HDD is magnetic.
Check yourself — tap to reveal
Answer in your head, then tap to see the model answer. Aim for 5 of 6 before moving to Learn.
✅ Ready for Learn?
Before moving on, make sure you can answer these without notes:
- What is the purpose of secondary storage? (Exact wording.)
- What are the three types, and an example of each?
- How does each type physically store data?
- Why is an HDD magnetic, not optical?
- How do you correctly compare solid-state and optical?
What is secondary storage?
Secondary storage
Storage that is NOT directly accessed by the CPU. Data must be sent to RAM first for processing. Non-volatile — data is kept without power. Used to store data permanently until deleted by the user.
Three types of secondary storage
Magnetic
How: Spinning platters with a read/write head. Electromagnet magnetises dots on the platter (1 = magnetised, 0 = demagnetised). Platters have tracks and sectors.
Examples: Hard Disk Drive (HDD).
Key: High capacity, cheap per GB, but has moving parts = vulnerable to damage.
Optical
How: Laser burns pits into the surface in a spiral track from the centre. Surface between pits = lands. Laser reads the pattern of pits and lands.
Examples: CD, DVD, Blu-ray.
Key: Cheap, portable, lower capacity. CD/DVD use red laser, Blu-ray uses blue laser.
Solid state
How: No moving parts. Uses transistors/semiconductor chips. Data is "flashed" onto cells. Control gate + floating gate transistors store binary.
Examples: SSD, USB flash drive.
Key: Fast, durable, lighter, silent, but more expensive per GB.
Comparison table
| Magnetic (HDD) | Optical (CD/DVD/Blu-ray) | Solid State (SSD/USB) | |
|---|---|---|---|
| Speed | Medium | Slowest | Fastest |
| Capacity | Highest (TB+) | Lowest (700MB–50GB) | High (TB available) |
| Cost per GB | Cheapest | Cheap per disc | Most expensive |
| Durability | Fragile (moving parts) | Can be scratched | Most durable (no parts) |
| Portability | Heavy/bulky | Very portable (thin disc) | Very portable (small) |
| Noise | Noisy (spinning) | Some noise | Silent |
| Moving parts? | Yes (platters + head) | Yes (spinning disc + laser) | No |
Quick check
Which storage type has NO moving parts?
Activity 1 — Magnetic, Optical or Solid State?
Press New Question.
Activity 2 — Best Storage for Scenario
Press New Question.
Activity 3 — Spot the Mistake
Press New Question.
Adaptive practice
Press New Question.
Cambridge-style questions
Press New Question.
Review quiz
Press New.
Exam traps
Memory triggers
🧠 "Magnetic = Moving"
Platters spin, head moves. Both start with M. Moving parts = fragile.
🧠 "Optical = eye (light)"
Optical comes from optics (light). Laser (light) reads pits and lands.
🧠 "Solid = Still"
Solid state has no moving parts. Solid and Still both start with S. Fast, quiet, durable.