Lesson 5

Computer Systems Lesson 5 of 5

Storing data long-term

RAM loses everything when the power goes off. Secondary storage keeps data permanently. But not all storage technology is the same - HDDs, SSDs, optical and flash each have different strengths, weaknesses, and ideal use cases.

35-45 minutes Core content Core GCSE CS content Interactive comparison

Real decisions

Netflix stores around 15 petabytes of video data. A hospital archive holds millions of patient scan images accumulated over decades. A gamer's laptop needs their 100 GB game to load in seconds. A student needs a USB stick that fits in a pocket and survives being washed.

These are all storage problems - but each has a different best solution. No single storage technology wins on every dimension.

This lesson: Understand how each storage technology works, what it is good at, and why engineers choose it for specific contexts.

Storage technologies

Four types of secondary storage

Select a technology to explore how it works and when to use it.

HDD

SSD

Optical

Flash

Hard Disk Drive (HDD)

How it works

An HDD stores data magnetically on spinning metal platters coated with magnetic material. A read/write head moves across the platters on an arm, magnetising tiny areas to store 1s and 0s. The platters spin at 5,400 or 7,200 RPM (rotations per minute). The head must physically move to the correct position - this mechanical movement is the main source of latency.

Characteristics

High capacity - 1 TB to 20 TB common

Low cost per GB - cheapest storage per byte

Slow - mechanical movement causes latency (5-10 ms seek time)

Fragile - spinning parts are vulnerable to shocks and drops

Noisy - audible when reading/writing

Best used for

Desktop backup drives where cost and capacity matter more than speed

Large-scale cold storage (data accessed rarely) in data centres

Network attached storage (NAS) for home media libraries

Not suited for laptops used on the move (fragile) or where boot speed matters

How the read/write head works

The arm pivots to position the head over the correct track. The platter spins until the right sector passes under the head. This mechanical wait is seek time - the HDD's main bottleneck.

Click a track to seek.

Solid State Drive (SSD)

How it works

An SSD stores data in NAND flash memory chips - no moving parts at all. Data is stored as electrical charge in floating-gate transistors. Without mechanical movement, access times drop from milliseconds to microseconds. NVMe SSDs connect directly to the CPU via PCIe lanes, bypassing the slower SATA interface used by older SSDs and HDDs.

Characteristics

Very fast - 500 MB/s (SATA) to 7,000 MB/s (NVMe PCIe 4.0)

Silent, no vibration - no moving parts

Shock resistant - suitable for laptops and portable devices

More expensive per GB than HDD, especially at large capacities

Limited write cycles - each cell degrades over time with heavy writes

Best used for

Operating system drives - fast boot times and program loading

Laptops and mobile computers - durable, silent, low power

Gaming - rapid level loading, texture streaming

Video editing workstations - sustained read/write throughput

Inside an SSD: floating-gate transistor

Cell cross-section

Control Gate

oxide insulator

Floating Gate

tunnel oxide

Source / Drain Channel

Sense Amplifier

Use the buttons below to program, erase, or read this cell. Watch the floating gate and bit value change.

Bit stored

Electrons trapped = bit 1
No electrons = bit 0
No moving parts = no seek time

Optical Storage (CD/DVD/Blu-ray)

How it works

Optical discs store data as microscopic pits and lands (flat areas) on a spiral track on the disc surface. A laser reads the difference in light reflection between pits and lands to decode 0s and 1s. Writable discs use a dye layer that the laser burns permanently (write-once) or a phase-change material that can be switched between states (rewritable).

Characteristics

Durable for archiving - pressed CDs can last 50+ years

Inexpensive per disc and highly portable

Write-once media is tamper-evident - useful for legal archives

Very slow compared to SSD or even HDD

Low capacity - CD 700 MB, DVD 4.7 GB, Blu-ray 25-100 GB

Declining use - many devices no longer include optical drives

Best used for

Distributing software, films, music (though mostly replaced by streaming)

Long-term archiving where tape is unavailable (hospitals, legal records)

Write-once audit trails where data must not be altered

Pits and lands: burn and read data

LAND = unburned dye = reflective
Laser bounces back strongly = read as 1

PIT = laser-burned dye = non-reflective
Laser scatters, no return = read as 0

Disc track - click any cell to burn a pit

Read laser position:

Bit output:

Click the white cells above to burn pits (they turn dark), then click Scan.

Flash Storage (USB / SD Card)

How it works

Flash storage uses the same NAND memory technology as SSDs but packaged for portability. USB drives connect via the USB interface; SD cards use a dedicated slot. Both are non-volatile and have no moving parts. The controller chip manages wear levelling to distribute writes evenly across memory cells, extending lifespan.

Characteristics

Extremely portable - small form factor, lightweight

No power required to retain data - non-volatile

Shock and water resistant (USB drives especially)

Slower than SSD - USB 2.0 especially is very slow

Limited write cycles - cheaper drives degrade faster

Easy to lose - small size is a security risk

Best used for

Transferring files between devices

Camera storage (SD cards)

Portable backup of important files

Not suitable for primary OS storage or heavy write workloads

Wear leveling: why it matters

Every NAND cell wears out after a limited number of writes (oxide layer degrades). Wear leveling is a controller technique that spreads writes across ALL cells evenly. Click Write 10 blocks repeatedly and watch the difference.

No wear leveling

Same cells hit every time

Dead: 0 / 16

With wear leveling

Controller spreads writes evenly

Dead: 0 / 16

Fresh Worn Very worn DEAD Writes so far: 0

Press "Write 10 blocks" to begin. Watch the left grid die while the right survives.

Side by side

Comparing all four technologies

Technology	Speed	Capacity	Cost/GB	Portable?	Volatile?	Moving parts?
HDD	Slow ~100 MB/s, 5-10 ms seek	Highest 1-20 TB	Lowest ~2p/GB	No (fragile)	No	Yes (platters spin)
SSD	Fastest 500-7000 MB/s, no seek	High 256 GB-8 TB	Medium ~8p/GB	Yes	No	No
Optical	Very slow ~10 MB/s (Blu-ray)	Low 700 MB-100 GB	Very low per disc	Yes	No	Yes (disc spins)
Flash (USB/SD)	Medium 10-400 MB/s (USB 3.x)	Medium 8 GB-2 TB	Medium ~10p/GB	Most portable	No	No

Scenarios: which storage would you choose?

Scenario 1 of 4

A hospital needs to archive patient scan images for 30 years. The images will rarely be accessed but must be preserved reliably. Which storage type is most appropriate?

HDD

SSD

Optical (Blu-ray)

USB Flash Drive

Scenario 2 of 4

A video editor needs storage for a workstation where large 4K video files must be read and written continuously at high speed. Which is most appropriate?

HDD

SSD (NVMe)

Optical (DVD)

USB Flash Drive

Scenario 3 of 4

A company wants to store 50 TB of infrequently accessed backup data at the lowest cost possible. Which storage type best meets this need?

HDD

SSD

Optical

Flash

Scenario 4 of 4

A photographer needs to transfer images from a camera to a laptop in the field. The storage must be small, shock-resistant and work without any cables. Which is most suitable?

HDD

SSD

Optical

SD Card (Flash)

Exam focus

Questions often ask you to "justify" a storage choice for a given context. Always reference two or three specific characteristics that match the scenario - for example: "An SSD is most appropriate because it has no moving parts (shock resistant for portable use), provides fast read speeds (important for loading the OS quickly), and is non-volatile (data is retained without power)." Answering with the name alone scores no marks.

Revision

Computer Systems Flashcards

27 key terms across all 5 lessons. Filter by topic, flip to reveal, mark as known.

Open flashcards

Lesson 4 Series overview Exam practice

Lesson 5 - Teacher Resources

Storing data long-term

Teacher mode (all pages)

Shows examiner notes on the Exam Practice page

Suggested starter (5 min)

Hold up a phone, a laptop and (if available) an old DVD or USB stick. Ask: "Where is data stored in each of these?" Take answers, then ask: "Are they all doing the same job?" Students quickly realise that a phone photo stored on a chip is different from a film on a disc. This surfaces the core question of the lesson - why do we have so many different types of storage - before any formal teaching begins.

Lesson objectives

1Describe how magnetic platters, tracks, sectors and the read/write head work together in an HDD, and explain why seek time and rotational latency slow it down.

2Explain how a NAND floating-gate transistor stores a binary value by trapping or removing electrons, and describe what wear leveling does and why it is needed.

3Describe how data is encoded as pits and lands on an optical disc and how a laser reads it back.

4Compare HDD, SSD, optical and flash storage across speed, capacity, cost per GB, portability, durability and volatility - and select and justify the most appropriate type for a given context.

Key vocabulary (board-ready)

Seek time

The time taken for the HDD read/write arm to move physically to the correct track on the magnetic platter.

Rotational latency

The additional delay while the platter spins until the correct sector is positioned beneath the read/write head.

Floating-gate transistor

The storage cell inside a NAND flash chip. Electrons are tunnelled into (write) or out of (erase) a floating gate layer to represent a 0 or 1.

Wear leveling

A firmware technique that distributes writes evenly across all NAND cells to prevent any single cell from exhausting its write cycle limit prematurely.

Pit / Land

The two surface states of an optical disc. A land is flat and reflects laser light; a pit is an indentation that scatters it. The transitions between them represent binary data.

Non-volatile

A storage type that retains data when power is removed. HDD, SSD, optical and flash are all non-volatile. RAM is volatile.

Cost per GB

The storage cost divided by capacity. HDD is cheapest (~£0.02/GB), SSD is more expensive (~£0.06-0.10/GB). This ratio is the single most important trade-off in context questions.

Suggested lesson plan

0-5 min: Starter activity above. Collect responses. Bridge into "today we learn exactly why".

5-20 min: HDD theory + seek simulation. Arm moves to tracks; students measure simulated seek time. Key terms: platter, track, sector, actuator arm, seek time, rotational latency.

20-35 min: SSD floating-gate sim, optical pit/land sim, NAND wear-leveling comparison. Higher: floating-gate mechanism, quantum tunneling concept. All: optical read/write process.

35-45 min: Comparison section. Students complete use-case scenario questions interactively. Reinforce: "state technology + property + link to scenario" structure.

45-55 min: Students visit the Exam Practice page. Less confident: MCQ only. More confident: MCQ + Written Questions A and B.

55-60 min: Exit tickets (see below). Collect answers on mini-whiteboards or verbally.

Discussion prompts

A 20 TB HDD costs £300. An equivalent SSD costs £2,000. The company runs 1,000 backups a year, each taking 4 hours on HDD vs 40 minutes on SSD. Is the speed saving worth the price difference? Work it out together.

If optical discs are so slow, why do hospitals still use Blu-ray discs for long-term patient records instead of SSD backup drives?

Explain wear leveling to the person next to you using this analogy: imagine a hotel corridor with 100 rooms. If you always put guests in the same 5 rooms the carpet wears out fast. What would the hotel manager do instead?

Every write you make to a cloud service shortens a NAND cell somewhere. At Google scale - billions of writes per second - who pays for the cells that fail? Does this change how you think about "free" cloud storage?

Common misconceptions

X"SSDs are always better than HDDs" - redirect: cost per GB. At 4+ TB, HDDs are 3-5x cheaper. In a context question, speed alone never scores full marks - cost must be addressed when capacity is large.

X"Flash drives are just small SSDs" - same NAND cells, but a simpler controller, slower USB interface, and lower endurance rating. A USB drive cannot replace an SSD as OS storage - it will fail within months of constant writes.

X"Optical storage is obsolete" - Blu-ray is actively used for long-term archives in the film industry, hospitals and cloud providers. Its write-once property makes it immune to ransomware when stored offline.

X"Non-volatile means it never fails" - non-volatile means data persists without power. All non-volatile media still has a finite lifespan: HDDs crash, NAND cells degrade, optical discs can delaminate.

X"Pits represent 1, lands represent 0" - the standard is that a transition between pit and land (or land and pit) represents a 1; no transition is a 0. Accept either in an exam as long as the student is consistent.

Exit ticket questions

Name the two mechanical delays that make an HDD slower than an SSD.

[2 marks - seek time + rotational latency]

Explain what wear leveling does and why it is needed in flash storage.

[2 marks - distributes writes / prevents single cell exhaustion]

A school wants to store 500 copies of a 4 GB software installer to hand out to students on removable media. Give one storage type and justify your choice.

[2 marks - state type + link property to scenario]

Describe how a laser reads data from a CD. Use the words pit and land.

[2 marks - reflection difference / sensor detects change]

Mark scheme - key written questions

Q: Explain why an HDD is slower than an SSD. [2]

The arm must physically move to the correct track (seek time) [1]
The platter must rotate until the sector is under the head (rotational latency) [1]

Q: Why does flash storage have a limited write lifespan? [2]

Each write/erase cycle degrades the oxide layer around the floating gate [1]
After enough cycles the cell can no longer hold charge / becomes a dead cell [1]

Context justification - model structure [3]

State the storage type [1]
Give a specific property of that type [1]
Link property explicitly to the scenario requirement [1]

Homework idea

Visit a major UK retailer (Scan.co.uk or Amazon UK). Find the current price per GB for: (1) a 4 TB HDD, (2) a 2 TB SATA SSD, (3) a 1 TB NVMe SSD, (4) a 256 GB USB 3.2 flash drive. Produce a table with your findings. Write one sentence explaining which has improved most in cost per GB over the past five years, and one sentence explaining when you would still choose an HDD over an SSD despite the speed difference.

Classroom tips

The HDD seek simulation has real pedagogical value - have students actually time how long it takes the arm to reach the outer vs inner tracks. The difference is immediately obvious and motivates the SSD comparison.

For the floating-gate transistor: don't get bogged down in quantum tunneling at GCSE. The key idea is "electrons in = stored 0, electrons out = stored 1." The mechanism just needs to be "controlled electrical force."

Differentiation: Less confident students do comparison table + use-case MCQ. More confident students write mark-scheme quality justifications for Questions A-C on the Exam Practice page before checking answers.

The wear-leveling simulation side-by-side comparison is very effective - students see immediately why a naive write approach destroys cells faster. Ask them to predict which grid will fail first before running the simulation.

Resources

Exam practice page (MCQ + written Qs) Worksheet 1 - Key terms and comparison table Worksheet 2 - Justification practice Worksheet 3 - Full 35-mark exam paper Set as class homework (coming soon)