Secondary Storage Technologies

At A-Level you need a thorough understanding of secondary storage — not just what the technologies are, but how they work at a physical level, their performance characteristics, and the trade-offs between them.

Why Secondary Storage?

Main memory (RAM) is volatile — it loses its contents when the power is switched off. Secondary storage is non-volatile and retains data permanently (until deliberately erased). It is used for long-term storage of programs and data.

Secondary storage is also much cheaper per gigabyte than RAM, but significantly slower.

Magnetic Storage — Hard Disk Drives (HDDs)

How HDDs Work

An HDD stores data on one or more spinning platters coated with a magnetic material. Data is read and written by a read/write head that floats just above the platter surface.

        Spindle
          │
    ┌─────┼─────┐
    │     │     │    ← Platters (spinning discs)
    │     │     │
    └─────┼─────┘
          │
    Read/write head ───── Actuator arm

Term	Definition
Platter	A circular magnetic disk; 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	An electromagnetic component that reads or writes data by detecting or changing the magnetic polarity of the platter surface
Actuator arm	Moves the read/write heads across the platters to the correct track

Performance Factors

Factor	Description
Seek time	Time for the head to move to the correct track
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 speeds: 5400, 7200, 10000, 15000 RPM

Advantages and Disadvantages

Advantages	Disadvantages
Large capacities (up to 20+ TB)	Moving parts — susceptible to mechanical failure
Low cost per GB	Slower than SSDs (especially random access)
Well-established technology	Heavier and larger than SSDs
Good for bulk/archive storage	Generates heat and noise

Solid-State Storage — SSDs and Flash Memory

How SSDs Work

SSDs use NAND flash memory — an array of floating-gate transistors arranged in a grid. Data is stored as electrical charge trapped in the floating gate.

State	Floating Gate Charge	Meaning
Programmed	Charge present	Binary 0 (in some conventions)
Erased	No charge	Binary 1

SSDs have no moving parts. Data is accessed electronically, not mechanically.

Types of NAND Flash

Type	Bits per Cell	Endurance	Speed	Cost
SLC (Single-Level Cell)	1	Highest	Fastest	Most expensive
MLC (Multi-Level Cell)	2	High	Fast	Moderate
TLC (Triple-Level Cell)	3	Moderate	Moderate	Cheaper
QLC (Quad-Level Cell)	4	Lowest	Slowest (of the four)	Cheapest

Secondary Storage Technologies (in depth)