🔒 Pro · Component 1 · 1.1 Input, Output and Storage
1.1.3b Secondary Storage Devices
OCR H446 · A Level Computer Science · ~13 min read
Notes
—
Video
—
Slides
—
Worksheet
—
Quiz
Why Secondary Storage?
Secondary storage provides persistent, non-volatile storage of data and programs. Unlike RAM, data is retained when power is removed. Secondary storage is slower than RAM but offers far greater capacity at lower cost per byte.
Hard Disk Drive (HDD)
An HDD stores data magnetically on spinning aluminium platters coated in a magnetic material.
How an HDD works:
Platters: one or more rigid aluminium discs rotate at high speed (typically 5,400 or 7,200 RPM).
Read/write head: floats on a cushion of air just above the platter surface, never physically touching it.
Magnetic encoding: the write head magnetises tiny regions of the platter surface to represent 0s and 1s. The read head detects changes in magnetic flux to read data.
Tracks, sectors, clusters: data is organised in concentric circles (tracks), each split into sectors (512 bytes or 4 KB). Groups of sectors form clusters (allocation units). The File Allocation Table (FAT) or NTFS keeps track of which clusters belong to which files.
Seek time: the time for the head to move to the correct track. Rotational latency: the time waiting for the correct sector to rotate under the head. Total = seek time + rotational latency + transfer time.
Characteristic
HDD
Capacity
High (1–20+ TB consumer drives)
Speed
Moderate (100–200 MB/s sequential)
Cost per GB
Low
Moving parts
Yes (fragile, noise, vibration sensitivity)
Power consumption
Higher
Solid State Drive (SSD)
An SSD stores data in NAND flash memory cells. There are no moving parts. Flash cells trap electrons in a floating gate to represent data.
NAND flash cell types:
SLC (Single-Level Cell): 1 bit per cell. Fastest, most durable, most expensive.
TLC (Triple-Level Cell): 3 bits per cell. Slower write speed, lower endurance, cheapest. Most consumer SSDs.
QLC (Quad-Level Cell): 4 bits per cell. Very high density, low endurance.
SSD controller and wear levelling:
Flash cells wear out after a finite number of write/erase cycles (program/erase cycles, PE cycles). The SSD controller uses wear levelling to spread writes evenly across all cells.
TRIM: an OS command that tells the SSD which blocks are no longer in use, allowing the controller to erase them in advance of future writes, maintaining performance.
Over-provisioning: SSDs reserve extra flash capacity (e.g. 7%–28%) not visible to the OS, used for wear levelling and bad block management.
Characteristic
SSD
Capacity
Moderate (250 GB–8 TB consumer)
Speed
Very high (SATA: ~550 MB/s; NVMe: 3,500–7,000+ MB/s)
Cost per GB
Higher than HDD
Moving parts
None (very durable, silent)
Power consumption
Lower
Optical Storage
Optical drives use a laser to read and write data on discs (CD, DVD, Blu-ray).
How it works:
Read-only (ROM): a laser reflects off the disc surface. Pits (indentations pressed into the disc) scatter light; lands (flat areas) reflect light. Transitions between pits and lands represent 1s; no transition represents 0s.
Recordable (R): a write laser heats a dye layer, creating permanent dark spots (simulating pits). Can only be written once.
Rewritable (RW): uses a phase-change alloy. A high-power laser melts the alloy to create amorphous regions (simulating pits); a lower-power laser recrystallises them to erase. Allows multiple write cycles.
Capacities:
CD: ~700 MB
DVD: ~4.7 GB (single layer), 8.5 GB (dual layer)
Blu-ray: ~25 GB (single layer), 50 GB (dual layer) — uses a shorter wavelength blue laser for finer detail
HDD vs SSD Comparison
Feature
HDD
SSD
Technology
Magnetic platters
NAND flash memory
Speed
Slower (mechanical seek time)
Much faster (no seek time)
Noise
Audible (spinning/seeking)
Silent
Shock resistance
Poor (moving parts)
Excellent (no moving parts)
Cost per GB
~£0.01–0.02
~£0.05–0.10
Lifespan concern
Mechanical failure, head crash
Wear-out of flash cells (limited PE cycles)
Best use
High-capacity archive / backup storage
OS drive, frequently accessed data, laptops
Exam tip: When comparing HDD and SSD, always address speed, moving parts, noise, cost per GB, durability, and best use case. These all appear in OCR H446 mark schemes.
Exam tip: For optical storage, know pit/land, read-only vs recordable vs rewritable, and why Blu-ray has higher capacity (shorter wavelength laser = finer pits = more data per unit area).
⚠ Common Mistakes
Saying SSDs use magnetic storage — SSDs use NAND flash (electronic charge in transistors), not magnetic.
Saying transitions from pits to lands represent 1s — in optical storage, it is the transition between pit and land (or land and pit) that represents a 1; staying on a pit or land represents 0.
Confusing seek time with transfer rate — seek time is the time to position the head; transfer rate is how fast data is read once positioned.
✓ Notes completed!
▶
Video coming soon
Click to advance · Arrow keys also work
Click slide or press arrow keys to navigate
✍
Worksheet — 1.1.3b Secondary Storage Devices
8 questions · 20 marks · instantly marked
Q1Describe how data is written to and read from a hard disk drive (HDD). Your answer should mention platters, tracks, the read/write head, and magnetic encoding.[4 marks]
✓ Mark scheme
Data is stored on one or more aluminium platters coated in magnetic material that spin at high speed (e.g. 7,200 RPM) [1]; data is organised in concentric tracks on each platter [1]; the read/write head, which floats above the platter surface without touching it, magnetises tiny regions in different orientations to write 1s and 0s [1]; to read, the head detects changes in the magnetic flux of the magnetised regions and converts them to electrical signals [1].
Q2Explain what is meant by 'seek time' and 'rotational latency' in an HDD, and explain why these do not affect an SSD.[4 marks]
✓ Mark scheme
Seek time is the time taken for the read/write arm to move the head to the correct track on the platter [1]; rotational latency is the time for the correct sector to rotate around to be under the read/write head [1]; SSDs have no moving parts — data is accessed electronically from flash memory cells using the SSD controller, so there is no physical movement and therefore no seek time or rotational latency [1]; this makes SSDs significantly faster for random access [1].
Q3Explain how a read-only optical disc (CD-ROM) stores and represents binary data using pits and lands.[3 marks]
✓ Mark scheme
The disc surface has a spiral track with alternating pits (indentations) and lands (flat regions) physically pressed into it during manufacture [1]; a low-power laser is directed at the spinning disc; lands reflect the laser beam strongly back to a photodetector, pits scatter the light reducing the reflected signal [1]; a transition between a pit and a land (or land to pit) represents a binary 1; remaining on a pit or land (no transition) represents a binary 0 [1].
Q4Explain what NAND flash memory is and describe how it stores data at the cell level. Include a comparison of SLC and TLC cells.[4 marks]
✓ Mark scheme
NAND flash memory is a type of non-volatile electronic storage that retains data without power [1]; each memory cell consists of a transistor with a floating gate; electrons are trapped in or removed from the floating gate to represent binary data — the quantity of stored charge is interpreted as data values [1]; SLC (Single-Level Cell) stores 1 bit per cell, is the fastest and most durable but most expensive per GB [1]; TLC (Triple-Level Cell) stores 3 bits per cell, providing higher capacity per chip at a lower cost per GB, but has lower write speed and endurance (fewer write/erase cycles before failure) [1].
Q5Explain what 'wear levelling' is and why it is needed in an SSD.[2 marks]
✓ Mark scheme
Flash memory cells degrade after a limited number of write/erase cycles (program/erase cycles), so without intervention frequently-written cells would fail first [1]; wear levelling is an algorithm in the SSD controller that distributes write operations evenly across all available flash memory blocks so no single area wears out prematurely, extending the overall lifespan of the SSD [1].
Q6A Blu-ray disc stores more data per unit area than a CD. Explain why, in terms of the laser used.[2 marks]
✓ Mark scheme
Blu-ray uses a shorter wavelength (405 nm blue-violet) laser compared to the red laser used in DVDs (650 nm) and infrared used in CDs (780 nm) [1]; a shorter wavelength can focus to a smaller spot, meaning pits and lands can be made smaller and packed more closely together, increasing the data density and storage capacity per layer [1].
Q7Explain what the TRIM command does in an SSD and why it is important for maintaining performance.[2 marks]
✓ Mark scheme
When files are deleted in an SSD, flash blocks are marked as unused but not immediately erased (unlike RAM). TRIM is an OS command sent to the SSD controller identifying which blocks are no longer in use [1]; the controller can then erase these blocks in advance (during idle time), so they are ready for fresh writes immediately rather than requiring an erase-then-write cycle, maintaining write speed and performance [1].
Q8Give two reasons why a business might choose to keep HDDs in addition to SSDs in their server infrastructure.[2 marks]
✓ Mark scheme
Any two of: HDDs have a much lower cost per GB, making them economical for large-capacity archival/backup storage where speed is less critical [1]; HDDs are available in much larger individual capacities (e.g. 20 TB vs typical SSD consumer capacities), useful for bulk data storage [1]; some data is accessed infrequently (cold data) so the speed advantage of SSDs is not needed [1].
Topic Quiz
1 of 15
You scored
out of 15
🎯
Mini Test — 1.1.3b Secondary Storage
10 questions · 10 marks · 10 minutes
5 MCQ + 5 short answer
⏱10:00
10 marks
Section A — Multiple Choice
Q1An SSD stores data using:
Q2In an HDD, 'rotational latency' refers to:
Q3A TLC NAND flash cell stores:
Q4Blu-ray discs have greater storage capacity than DVDs because:
Q5The main reason an SSD is faster than an HDD for random file access is:
Section B — Short Answer
Q6State what 'wear levelling' does in an SSD.
Mark schemeWear levelling distributes write/erase operations evenly across all flash memory blocks to prevent any single area from wearing out prematurely, extending the SSD's lifespan. [1 mark]
Q7State what a 'track' and a 'sector' are in the context of an HDD.
Mark schemeA track is a concentric circle on the platter surface where data is stored [1]; a sector is a fixed-size subdivision of a track (e.g. 512 bytes) — the smallest addressable unit on the disk [1]. (1 mark for each correct definition)
Q8In optical storage, what does a transition between a pit and a land represent?
Mark schemeA transition between a pit and a land (or land and pit) represents a binary 1. No transition (remaining on a pit or land) represents a binary 0. [1 mark]
Q9State one advantage of an HDD over an SSD for backup storage.
Mark schemeHDDs have a lower cost per GB, making them more economical for large-capacity backup storage [1] / HDDs are available in larger individual capacities (e.g. 20+ TB) [1]. (accept either)
Q10State what SLC stands for in the context of NAND flash memory and state one advantage over TLC.
Mark schemeSLC = Single-Level Cell (1 bit per cell) [1]; advantage: faster write speed and greater endurance (more write/erase cycles before failure) than TLC [1].