I.9 Storage Media and Their Types

⬆️ Back to: P-I.I. Introduction to Computing

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Page Outline — click a heading to jump. Past-paper questions sit in the last section.

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Exam pattern (2016–2026): 2025 Q2(b) storage comparison (6) · 2026 MCQ Q1 (SSD) · SSD part of 2021 Q3(a). Asked two years running — rising.

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1. Definitions

• Storage medium: the material that holds the data (disc, platter, tape, flash chip).
• Storage device: the hardware that reads/writes it (drive, reader).
  • DVD disc = medium; DVD drive = device.
  • Tape cartridge = medium; tape drive = device.
  • SD card = medium; card reader = device.
  • 🧠 If you can carry it to another reader, it is the medium.
• Four technology types: magnetic, optical, solid-state, cloud.

2. Memory / Storage Hierarchy

• Going down: speed falls, capacity rises, cost per GB falls.

• Scale: a 10 ms HDD access is about 10 million times slower than a register.

3. Classification Axes (each independent)

• Volatility:
  • Volatile: RAM, cache, registers.
  • Non-volatile: HDD, SSD, optical, tape, ROM, cloud.
• Access method:
  • Sequential: magnetic tape only.
  • Random/direct: HDD, SSD, optical, RAM.
• Role: primary (RAM, cache — CPU uses directly) vs secondary (via I/O controller).
• Portability:
  • Removable: USB drive, SD card, external disk, discs, tape.
  • Fixed: internal HDD/SSD, RAM, cache, registers.
• Purpose: working storage vs backup (a spare copy for recovery).

4. Magnetic Storage

4.1 HDD

• Spinning platters (5,400–7,200 rpm) and a moving head.
• Pros: large, cheapest per GB for active use.
• Cons: 5–10 ms seek time; head crash on shock; wears out.

4.2 Magnetic tape

• Sequential only — must spool to the data.
• Pros: lowest cost per TB, about 30-year shelf life, offline (ransomware-proof).
• Cons: access in seconds to minutes; unfit for daily use.
• Use: archives, bank records, national data backups (LTO cartridges).

5. Optical Storage

• Laser reads pits and lands on a spinning disc.
• CD: about 700 MB.
• DVD: about 4.7 GB (single layer).
• Blu-ray: 25 GB+ per layer — uses a 405 nm blue-violet laser (DVD uses 650 nm red), so pits are smaller.
• Pros: cheap, portable, immune to magnetism.
• Cons: slowest of the three, small capacity, scratches easily.

6. Solid-State Storage

• Charge trapped in NAND flash cells; no moving parts.
• SSD:
  • Access about 0.05–0.1 ms (about 100× faster than HDD).
  • Transfer: SATA SSD ≈ 550 MB/s; NVMe SSD ≈ 3,000–7,000 MB/s (HDD ≈ 100–160 MB/s).
  • Shock-proof, silent, low power.
  • Limit: finite write cycles (rated in TBW); wear-levelling spreads writes.
• USB flash drive: plugs into USB; moves files between machines.
• Memory cards (SD/microSD): cameras, phones.

7. Cloud Storage

• Data on a provider's remote servers (Google Drive, Dropbox, iCloud, Amazon S3).
• Pros: no local media, access anywhere, off-site copy.
• Cons: needs Internet; relies on a third party.
• Strictly a delivery model — data still lands on HDDs/SSDs somewhere.
• CSS angle: raises data-sovereignty questions (Personal Data Protection Bill, cloud and data-localisation policy).

8. Comparison of the Technologies

9. Backup Memory

• Defined by purpose, not technology: a spare copy for recovery.
• Common media: LTO tape, external HDD/SSD, cloud backup, archival discs.
• RAID is not a backup: it survives a disk failure, but not deletion or ransomware.
• 3-2-1 rule: 3 copies, 2 media types, 1 off-site.
• CSS angle: NADRA, FBR and SBP need offline, off-site copies for disaster recovery.

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10. ⚠️ MCQ Traps

• Tape is sequential only; everything else here is random/direct access.
• SSD is faster because it has no moving parts, not because it is "newer".
• Optical resists magnets but scratches.
• Down the hierarchy: speed falls, capacity rises.
• Solid-state ≠ volatile: SSDs and USB drives keep data.
• Portable is about mobility, not speed or volatility.
• Backup is a purpose, not a technology.

11. ✅ Self-Test Questions (with Answers)

Q1. Compare magnetic, optical and solid-state storage.

• Use the table in section 8.
• Root cause: how a bit is stored (magnetism, pits, trapped charge).

Q2. Why is tape unsuitable for frequent access?

• Sequential: must spool to the data (seconds to minutes).
• Constant shuttling wears tape and drive.
• But ideal for: write-once, read-rarely archives — cheapest per TB, offline.

Q3. What single property makes SSDs faster and tougher than HDDs?

• No moving parts.
  • No seek time or rotational delay → about 0.05–0.1 ms vs 5–10 ms.
  • No head crash → shock-proof.
• Caveat: finite write cycles; unpowered SSDs can lose charge over many years.

Q4. Storage medium vs storage device?

• Medium stores; device accesses.
• Examples: DVD vs DVD drive; tape vs tape drive; SD card vs card reader.

Q5. Four types of storage by technology?

• Magnetic, optical, solid-state, cloud (with one example each).

Q6. Order registers, cache, RAM, secondary storage by speed.

• Registers > Cache > RAM > Secondary.
• Capacity rises in the opposite direction; cost per GB falls.

Q7. Classify HDD, SSD, RAM, tape by volatility.

• Only RAM is volatile; HDD, SSD and tape are non-volatile.

Q8. Which is sequential, which random access?

• Tape = sequential; HDD, SSD, optical, RAM = random/direct.

Q9. Two examples each of portable and fixed memory.

• Portable: USB drive, SD card.
• Fixed: internal SSD/HDD, RAM.

Q10. What is backup memory and what media are used?

• A spare copy for recovery — tape, external drives, cloud, archival discs.
• Mention the 3-2-1 rule and that RAID is not a backup.

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12. 📜 Past-Paper Questions & MCQs (CSS 2016–2026)

2025 · Paper-I · Q2(b) · 6 marks

Q. Compare different data storage devices in terms of speed, size, access time and cost.

Quick revision and exam-ready model

Quick revision — key points

• Registers/cache fastest but tiny and costly
• RAM is volatile and fast
• SSD persistent with low latency
• HDD slower/cheaper; tape for archives

Exam-ready answer — structured

Introduction / direct answer: CPU registers/cache: extremely fast, tiny capacity, lowest latency and high cost per byte; volatile.

Main explanation

• RAM: nanosecond-scale access, usually gigabytes, fast and volatile. NVMe SSD: persistent solid-state storage, often hundreds of GB to TB, microsecond-scale access, high throughput and moderate cost/GB.
• SATA SSD: persistent flash storage, generally slower than high-end NVMe but far faster random access than HDD. HDD: persistent, large TB capacity, mechanical millisecond-scale seek latency and comparatively low cost/GB.
• Optical discs/tape: removable or archival media; typically slower access, often with low archival cost per stored byte at scale. Rule: distinguish access latency from sequential transfer speed, and specify that actual price/speed varies by device generation.
• Use RAM for active work, SSD for responsive daily computing and HDD/tape for bulk or archival storage.

MCQs

2026 P-I · Q1. A company wants to upgrade its storage media for the fastest possible data access. Which one is the best option?

• (A) Hard Disk Drive (HDD)
• (B) Optical Drive
• (C) Solid State Drive (SSD) ✅
• (D) None of these
• Answer: (C) SSD.
• Why: an SSD has no moving parts, so its access time is about 0.1 ms, against about 5–10 ms for an HDD; optical drives are slower still.