Secondary storage: optical, magnetic and solid-state
Secondary storage provides permanent (non-volatile) storage for the operating system, programs and data. Unlike RAM, data is retained when the power is off. OCR J277 requires comparison of three technology types across five criteria: capacity, speed, durability, cost and reliability.
Why secondary storage is needed
- RAM is volatile and loses data when power is off.
- RAM is small and expensive.
- Secondary storage retains data permanently and provides large capacity at low cost.
- The OS, installed programs and user files all live in secondary storage.
The three types
1. Magnetic storage (Hard Disk Drive — HDD)
- Data stored as magnetic patterns on spinning metal platters.
- A moving read/write head accesses data as platters spin (typically 5,400–7,200 RPM).
| Criterion | Magnetic (HDD) |
|---|---|
| Capacity | Very high (up to 20+ TB for consumer drives) |
| Speed | Moderate (~100–200 MB/s sequential; slow random access due to seek time) |
| Durability | Low (moving parts — vulnerable to shocks and vibration; head crash risk) |
| Cost | Low per GB (cheapest option) |
| Reliability | Lower (mechanical failure; limited lifespan ~3–5 years) |
- Used for: desktop computers, NAS (network-attached storage), data centres needing bulk storage.
2. Solid-State Drive (SSD)
- No moving parts; data stored in flash memory chips (NAND flash).
- Electric charges in transistors represent 0s and 1s.
| Criterion | Solid-State (SSD) |
|---|---|
| Capacity | High (128 GB – 4 TB consumer; larger enterprise) |
| Speed | Very fast (500 MB/s SATA SSD; 3,500+ MB/s NVMe SSD) |
| Durability | High (no moving parts; not affected by vibration or shock) |
| Cost | Higher per GB than HDD |
| Reliability | High (but limited write cycles — can wear out after many writes) |
- Used for: laptops, tablets, smartphones, modern desktops for OS and programs.
3. Optical storage (CD, DVD, Blu-ray)
- Data stored as pits and lands on a reflective disc; a laser reads the pattern.
- Pits = 0; lands = 1 (or the transition between them).
| Criterion | Optical (CD/DVD/Blu-ray) |
|---|---|
| Capacity | Low (CD: 700 MB; DVD: 4.7 GB; Blu-ray: 25–128 GB) |
| Speed | Slow (typical DVD: ~11 MB/s) |
| Durability | Variable (discs can scratch; sensitive to UV; no moving parts beyond drive) |
| Cost | Very low per disc |
| Reliability | Low-medium (scratched discs unreadable; degradation over time) |
- Used for: distributing software/media, archiving, music/films.
Comparison table
| Magnetic (HDD) | Solid-State (SSD) | Optical | |
|---|---|---|---|
| Capacity | Very high | High | Low |
| Speed | Moderate | Very fast | Slow |
| Durability | Low (moving parts) | High | Medium |
| Cost/GB | Lowest | Medium | Very low/disc |
| Reliability | Lower | Higher | Medium |
Common OCR exam mistakes
- Saying "SSDs have no moving parts so they never fail" — SSDs have limited write cycles and can wear out.
- Confusing capacity (total storage) with speed (read/write rate) — OCR questions often require you to discuss both separately.
- Saying optical discs are durable — they are vulnerable to scratches, UV damage and degradation.
- Forgetting the mechanism: HDDs use magnetic patterns on spinning platters; SSDs use flash memory; optical uses laser + pits/lands.
AI-generated · claude-opus-4-7 · v3-ocr-computer-science