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This lesson covers secondary storage technologies: magnetic (HDD), optical (CD, DVD, Blu-ray), solid state (SSD, flash) and cloud storage. For the OCR H446 exam you must understand how each technology works at a physical level, and compare them in terms of capacity, speed, portability, durability and cost.
Primary storage (RAM) is volatile — it loses its contents when the power is switched off. Secondary storage is non-volatile, meaning it retains data permanently (until deliberately erased). It is used for long-term storage of programs, files and the operating system.
Secondary storage is also much cheaper per gigabyte than RAM, although significantly slower.
An HDD stores data on one or more spinning platters coated with a ferromagnetic material. Data is read and written by a read/write head that floats just above the platter surface on a cushion of air.
Spindle
|
+-----+-----+
| | | <-- Platters (spinning discs)
| | |
+-----+-----+
|
Read/write head ----- Actuator arm
| Term | Definition |
|---|---|
| Platter | A circular magnetic disc; data is stored on both surfaces |
| Track | A concentric ring on a platter surface |
| Sector | A segment of a track — the smallest addressable unit (typically 512 bytes or 4 KB) |
| Cylinder | The set of tracks at the same position on all platters |
| Read/write head | Detects or changes the magnetic polarity of the platter surface to read or write data |
| Actuator arm | Moves the heads across the platters to the correct track |
| Factor | Description |
|---|---|
| Seek time | Time for the head to move to the correct track (typically 5-15 ms) |
| Rotational latency | Time for the platter to rotate so the correct sector is under the head |
| Data transfer rate | Speed at which data is read from or written to the platter |
| RPM | Revolutions per minute — faster spin = lower latency. Common: 5400, 7200, 10000 RPM |
| Advantages | Disadvantages |
|---|---|
| Very large capacities (up to 20+ TB) | Moving parts — susceptible to mechanical failure and shock damage |
| Low cost per GB | Slower than SSDs (especially random access) |
| Well-established, reliable technology | Heavier and larger than SSDs |
| Good for bulk/archive storage | Generates heat and noise |
Data is stored as a spiral track of pits (depressions) and lands (flat areas) on the disc surface. A laser reads the disc by detecting the difference in reflection between pits and lands.
| Format | Capacity (single layer) | Laser Wavelength | Laser Colour |
|---|---|---|---|
| CD | ~700 MB | 780 nm | Infrared |
| DVD | 4.7 GB | 650 nm | Red |
| Blu-ray | 25 GB | 405 nm | Blue-violet |
A shorter wavelength laser can focus on smaller pits, allowing higher data density on the disc.
| Type | Description |
|---|---|
| ROM (Read-Only) | Data stamped at manufacture; cannot be changed. Used for commercial distribution |
| R (Recordable) | Can be written to once using a laser to burn marks in a dye layer |
| RW (Rewritable) | Can be erased and rewritten multiple times using a phase-change material |
| Advantages | Disadvantages |
|---|---|
| Very cheap per disc | Small capacity compared to HDD/SSD |
| Highly portable | Slow read/write speeds |
| Good for distribution (music, films, software) | Easily scratched or damaged |
| Long archival life (pressed discs) | Becoming obsolete for many purposes |
SSDs use NAND flash memory — arrays of floating-gate transistors. Data is stored as electrical charge trapped in the floating gate of each transistor.
| State | Charge | Meaning |
|---|---|---|
| Programmed | Charge present in floating gate | Binary 0 |
| Erased | No charge in floating gate | Binary 1 |
SSDs have no moving parts — data is accessed entirely electronically.
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