Secondary Storage Types: Magnetic, Optical, and Solid-State

This lesson covers the three main types of secondary storage — magnetic, optical, and solid-state — as required by OCR J277 Section 1.2.2. You need to understand how each type works, its advantages and disadvantages, and when each is appropriate.

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Why Is Secondary Storage Needed?

RAM is volatile — it loses all data when the computer is turned off. Secondary storage provides non-volatile, permanent storage so that data, files, and programs are preserved when the computer is powered down.

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Type 1: Magnetic Storage

How It Works

Magnetic storage uses magnetised particles on a spinning surface to store binary data. The read/write head moves across the spinning platters to read or write data.

Key Example: Hard Disk Drive (HDD)

Feature	Detail
Mechanism	Spinning metal platters with magnetic coating; read/write head on an arm
Capacity	Very large — typically 500 GB to 20 TB
Speed	Moderate — typically 100-200 MB/s
Cost	Low cost per gigabyte
Durability	Contains moving parts — vulnerable to damage if dropped or shaken

Other Magnetic Storage

• Magnetic tape — used for long-term backup and archiving. Very high capacity and low cost, but very slow (sequential access — must read through the tape in order to find data).

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Type 2: Optical Storage

How It Works

Optical storage uses a laser to read and write data. Data is stored as a pattern of pits (indentations) and lands (flat areas) on the surface of a disc. The laser reflects differently off pits and lands, which the drive interprets as binary 0s and 1s.

Key Examples

Disc Type	Capacity	Writable?
CD-ROM	~700 MB	Read-only (pre-pressed)
CD-R	~700 MB	Write once, read many
CD-RW	~700 MB	Rewritable
DVD	~4.7 GB (single layer)	Various (ROM, R, RW)
Blu-ray	~25 GB (single layer)	Various (ROM, R, RE)

Characteristics of Optical Storage

Feature	Detail
Capacity	Low to moderate (700 MB to 128 GB for multi-layer Blu-ray)
Speed	Slow compared to HDDs and SSDs
Cost	Very low per disc
Durability	Can be scratched; sensitive to direct sunlight
Portability	Very portable and lightweight

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Type 3: Solid-State Storage

How It Works

Solid-state storage uses electronic circuits (specifically, NAND flash memory) to store data. There are no moving parts — data is stored by trapping electrons in floating-gate transistors.

Key Examples

Device	Typical Capacity
SSD (Solid-State Drive)	120 GB to 8 TB
USB flash drive	4 GB to 1 TB
SD card / microSD	2 GB to 1 TB

Characteristics of Solid-State Storage

Feature	Detail
Speed	Very fast — typically 500-7,000 MB/s (NVMe SSDs)
Capacity	Moderate to large
Cost	Higher cost per gigabyte than HDDs
Durability	Very durable — no moving parts, resistant to shock
Power	Low power consumption
Noise	Silent operation
Lifespan	Limited number of write cycles (but sufficient for most uses)

OCR Exam Tip: A common comparison question asks about HDD vs SSD. Focus on: SSDs are faster and more durable (no moving parts) but cost more per GB; HDDs offer higher capacity at lower cost but are slower and fragile.

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Secondary Storage Family Tree

The diagram below organises the main secondary storage technologies by category:

graph TD
    SS["Secondary Storage<br/>(non-volatile)"]
    SS --> MAG["Magnetic"]
    SS --> OPT["Optical<br/>(laser, pits and lands)"]
    SS --> SOL["Solid-State<br/>(NAND flash)"]
    MAG --> HDD["HDD<br/>(spinning platters)"]
    MAG --> TAPE["Magnetic tape<br/>(archive, sequential)"]
    OPT --> CD["CD<br/>~700 MB"]
    OPT --> DVD["DVD<br/>~4.7 GB"]
    OPT --> BR["Blu-ray<br/>~25 GB"]
    SOL --> SSD["SSD"]
    SOL --> USB["USB flash drive"]
    SOL --> SD["SD / microSD card"]

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Comparing the Three Types

Feature	Magnetic (HDD)	Optical (CD/DVD/Blu-ray)	Solid-State (SSD/Flash)
Speed	Moderate	Slow	Fast
Capacity	Very high	Low-moderate	Moderate-high
Cost per GB	Low	Very low per disc	High
Durability	Fragile (moving parts)	Scratch-prone	Very durable
Portability	Moderate	Very portable	Very portable
Power use	Moderate	Low	Very low
Noise	Audible (spinning)	Audible (spinning)	Silent

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Summary

• Secondary storage is needed because RAM is volatile (OCR J277 1.2.2).
• Magnetic storage (HDD): high capacity, low cost, but fragile and moderate speed.
• Optical storage (CD/DVD/Blu-ray): portable and cheap, but low capacity and slow.
• Solid-state storage (SSD/flash): fast and durable, but higher cost per GB.
• Each type has specific use cases depending on the requirements of the situation.

Key Vocabulary: magnetic, optical, solid-state, HDD, SSD, flash memory, NAND, pit, land, platter, read/write head, sequential access.

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Extended Worked Example: HDD vs SSD in the Same Laptop

A Year 11 student, Leo, inherits his older sister's 2018 laptop. The machine has an Intel i5 processor, 8 GB of RAM and a 1 TB HDD. It takes 95 seconds to boot, Chrome takes 12 seconds to open, and opening a large coursework document in Word takes another 8 seconds. Leo can afford to upgrade one component and the IT shop offers him a choice: more RAM (to 16 GB) or a 500 GB SSD replacing the HDD. This worked example traces what happens with each choice.