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This lesson pulls the storage and memory strands together: it sets every store on a single hierarchy from registers down to offline media, then compares the secondary technologies across the criteria examiners test — capacity, speed, cost/GB, durability, portability and power — so you can justify a storage choice for any scenario.
This lesson develops OCR H446 section 1.1.2 (memory and storage) by comparing storage technologies and situating them in the memory/storage hierarchy. It compares magnetic, solid-state, optical and remote storage on capacity, speed (access time and transfer rate), cost per gigabyte, durability, portability and power consumption, and applies these criteria to choose storage for a scenario. It builds directly on the secondary-storage lesson (operating principles) and the primary-storage lesson (RAM, cache, registers, the hierarchy), and links to virtual memory in the software-systems memory-management lesson.
Before comparing devices, fix the big picture: every store the spec mentions sits on one continuous spectrum. As you move down it, speed and cost-per-bit fall while capacity rises; as you move up, the opposite. No technology is simultaneously fast, large and cheap, so systems layer them and shuttle data upward on demand.
flowchart TB
REG["Registers<br/>bytes · ~1 cycle · fastest · dearest per bit"]
CACHE["Cache L1/L2/L3 (SRAM)<br/>KB–MB · a few cycles"]
RAM["Main memory / RAM (DRAM)<br/>GB · ~100 ns · volatile"]
SEC["Secondary storage (SSD / HDD)<br/>TB · ms · non-volatile"]
OFF["Offline / archive (optical, tape, removable, cloud)<br/>huge · seconds–minutes to mount · cheapest per bit"]
REG --> CACHE --> RAM --> SEC --> OFF
OFF -. "data fetched upward on demand" .-> SEC
SEC -. "loaded into RAM to run" .-> RAM
Two boundaries on this diagram matter most. The line between RAM and secondary storage is the volatility boundary — everything above it is volatile and lost at power-off; everything below survives. The line between secondary and offline/archive storage separates media that are always connected and ready (your SSD/HDD) from media that must be mounted, inserted or downloaded before use (a Blu-ray, a tape, a cloud bucket). This lesson concentrates on the lower three tiers, because that is where "compare and choose" questions live; the upper tiers were covered in primary storage.
| Criterion | What it means | Why it matters |
|---|---|---|
| Capacity | How much data the device holds | Bulk archives need TB; a boot drive needs less |
| Speed | Access time (delay before transfer) + transfer rate (sustained throughput) | Editing/gaming need fast random access; archiving tolerates slow |
| Cost per GB | Device price ÷ capacity | Dominates large-scale and budget decisions |
| Durability | Resistance to shock, wear, water; expected lifespan | Field/portable use; long-term archive |
| Portability | Physical size, weight and whether it needs mains power | Carrying data between sites |
| Power | Energy drawn in use and idle | Laptops, data-centre running costs, battery devices |
Note the two halves of speed: a device can have a high transfer rate yet poor access time. An HDD streams large files reasonably fast (good transfer rate) but is slow to start each access (poor random access time) — the distinction that decides many scenarios.
The art of these questions is realising that the criteria conflict, so there is rarely a single "best" device — only the best for a given priority. A few of the most important tensions:
Recognising which tension a scenario turns on is what tells you which criterion should dominate — and naming that explicitly is what examiners reward.
| Technology | Typical capacity | Notes |
|---|---|---|
| HDD | 500 GB – 20+ TB | Highest per single device; bulk storage |
| SSD | 120 GB – 8 TB (consumer) | Common: 256 GB, 512 GB, 1 TB, 2 TB |
| CD / DVD / Blu-ray | 0.7 / 4.7 / 25 GB | Fixed per disc; low |
| USB flash drive | 2 GB – 2 TB | Small form factor |
| SD card | 2 GB – 1 TB | Cameras, phones, drones |
| Cloud | "Unlimited" (pay for more) | Limited by subscription/provider |
A capacity subtlety worth carrying into calculations is the decimal-vs-binary prefix mismatch (from data representation). Drive makers advertise capacity in decimal units — a "1 TB" drive holds 1012 bytes — but operating systems often report capacity in binary units (a tebibyte, 240≈1.0995×1012 bytes), which they may label "TB". The same physical drive therefore appears as "1 TB" on the box but about "0.91 TB" in the OS — not missing space, just two different definitions of the prefix. Being aware of this avoids both confusion and arithmetic slips when a question mixes the conventions.
| Technology | Sequential read | Random access | Notes |
|---|---|---|---|
| HDD | 80–250 MB/s | Slow (seek 5–15 ms) | Mechanical head/platter latency |
| SSD (SATA) | 400–550 MB/s | Very fast (~0.1 ms) | Capped by the SATA interface |
| SSD (NVMe) | 2,000–7,000+ MB/s | Extremely fast | Uses PCIe lanes |
| CD / DVD / Blu-ray | 1–54 MB/s | Slow | Spin-up + single laser |
| USB flash drive | 5–400 MB/s | Moderate | Depends on USB version |
| Cloud | = internet speed | = internet speed | Network-bound, variable |
| Technology | Size / weight | Power | Portability |
|---|---|---|---|
| 3.5" HDD | Large, heavy | Mains | Low |
| 2.5" / portable HDD | Medium | USB-powered | Moderate (but fragile) |
| Portable SSD | Small, light | USB-powered | High |
| USB flash / SD | Tiny | Slot/bus-powered | Very high |
| Optical disc | Thin disc, but needs a drive | Drive needs power | Disc high; reader low |
| Cloud | No device to carry | Needs a network | Highest — reach it from any device |
Portability is more than size: a 3.5-inch HDD also needs mains power, so it is not truly "grab and go", whereas a bus-powered SSD or a USB stick draws all it needs from the host's port. And the most "portable" option of all is cloud, where there is no physical object to carry — you reach the data from whatever device is to hand — at the price of needing connectivity. Matching this to a scenario means asking not just "how big is it?" but "does it need its own power?" and "must the user carry a reader?".
| Technology | Moving parts | Shock resistance | Power draw | Typical lifespan |
|---|---|---|---|---|
| HDD | Yes (platters, heads) | Poor (head crash) | Higher (motor) | ~3–5 years |
| SSD | No | Excellent | Low | 5–10 years (write-limited) |
| Optical | Spins in a drive | Moderate (can shatter/scratch) | Low | Decades (pressed) |
| USB / SD | No | Good | Very low | 10+ years if lightly written |
| Cloud | N/A (provider) | N/A | N/A (client just needs network) | Indefinite (provider-dependent) |
Power is an examinable criterion in its own right and links durability to use-case. An HDD's spinning motor draws meaningfully more power than a flash device, which matters in two opposite settings: in a laptop or phone it shortens battery life, and in a data centre running thousands of drives it raises the electricity (and cooling) bill enormously — one reason large operators weigh SSDs' lower power against their higher purchase cost. For a battery device the calculus is clear-cut: flash's near-zero idle power and absence of a motor make it the only sensible choice, which is why no phone has ever contained a hard disk.
Rather than quote prices that date quickly, learn the ordering, which is stable: HDD is cheapest per GB, SSD is several times dearer, optical is cheap per disc but tiny in capacity, USB/SD carry a small-form-factor premium, and cloud is an ongoing per-month cost that can exceed buying a drive over a few years.
Cost per GB=capacity in GBdevice price.
For example, a £90, 4 TB HDD costs 90/4000≈£0.0225 per GB, whereas a £90, 1 TB SSD costs 90/1000=£0.09 per GB — about four times more per gigabyte, which is precisely the trade-off behind "HDD for bulk, SSD for speed".
"Compare/choose" questions often hide a calculation that quantifies the justification. Three patterns recur.
How many single-layer Blu-ray discs (25 GB) are needed to back up a 1 TB drive? Working in decimal GB:
25 GB/disc1000 GB=40 discs.
Forty discs to mirror one commodity hard drive instantly shows why optical is hopeless for bulk backup and belongs to distribution and archive instead.
How long to back up 500 GB over each medium? Using time = size ÷ transfer rate:
The cloud figure (note the careful bits-to-bytes conversion — a classic exam trap) makes the bandwidth limitation of cloud for large transfers vivid, and explains why cloud suits ongoing sync of modest files rather than bulk initial backup.
Cloud's recurring cost can quietly overtake local storage. Compare a £90 one-off 4 TB HDD with cloud at £8/month for the same 4 TB over three years:
cloud=8×12×3=£288,vs HDD=£90 (one-off).
Over three years the cloud costs more than three times the drive — though that buys offsite safety, remote access and no maintenance. Quantifying the trade-off, rather than just asserting "cloud has ongoing costs", is exactly what lifts an evaluation into the top band.
The core exam skill is matching technology to scenario and saying why. Each example states the dominant need, the pick, and the rejected alternatives.
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