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Hardware, Memory & Number Systems

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high importance7 formulas⚡ 14 shortcuts6 subtopics
OverviewLearn (6)Formulas (7)Examples (5)Practice (97)
All subtopics·Subtopic 6 of 6

Storage and backup media

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⏱ 4 min read🧩 5 question types🎯 16 practice Q
The idea in one minute

Primary memory (RAM and cache) is fast but small. Secondary storage is large, permanent and cheaper per GB. Secondary storage comes in three kinds: magnetic, optical and flash. Exams ask capacities, the HDD and SSD difference, and backup methods.

01

Primary and secondary storage

Primary memory (RAM and cache) is fast but small. RAM is also volatile. Secondary storage is large and permanent. It costs less per GB. Everything below is secondary storage.

02

The three technologies

FamilyHow it storesExamples
MagneticMagnetised spots on spinning platters or tapeHDD, old floppy, magnetic tape (LTO)
OpticalTiny pits read by a laserCD, DVD, Blu-ray
FlashMemory chips with no moving partsSSD, pen drive, memory card, phone storage
03

Optical disc capacities

This row is the exam favourite.

DiscCapacity (single layer)Laser
CD700 MBInfrared or red
DVD4.7 GB (dual layer about 8.5 GB)Red
Blu-ray25 GB (dual layer about 50 GB)Blue-violet

The order is CD (0.7 GB), then DVD (4.7 GB), then Blu-ray (25 GB). A blue-violet laser has a shorter wavelength. So it reads tighter pits and stores more data.

Tip: Remember "700, 4.7, 25". A CD holding 4.7 GB should feel wrong at once.

04

HDD and SSD

HDDSSD
PartsSpinning magnetic platters and moving headsFlash chips, no moving parts
SpeedSlower (heads must seek)Much faster
Shock and noiseSensitive, some noiseShock-resistant, silent
Cost per GBCheaperCostlier

Rule: "No moving parts" in a question means SSD.

05

Choosing a medium

  • Need speed, silence and shock resistance: SSD.
  • Need cheap bulk storage: HDD.
  • Need to keep archives for years at low cost: magnetic tape.
  • Need to carry small files: pen drive or memory card.
06

Access methods

  • Random (direct) access: jump straight to any address. RAM, HDD and SSD work this way.
  • Sequential access: read in order, like a cassette. Magnetic tape works this way.

Tape is cheap. So it suits archival backups, not daily use.

07

Backup ideas

  • Cloud storage or backup: your files are copied to internet servers. You can reach them from anywhere.
  • Tape libraries: banks and research labs still archive petabytes on tape.
  • The 3-2-1 habit: three copies, on two media, with one copy off-site.
08

Small facts exams quote

  • A floppy disk stored 1.44 MB. It is obsolete but still asked.
  • A memory card is flash storage for cameras and phones (SD, microSD).
  • Blu-ray gets its name from the blue-violet laser.
  • Formatting prepares a disk's file system. It does not add capacity.
  • Everyday anchors: an MP3 song is about 4 to 8 MB. A phone holds about 64 to 256 GB.
09

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common5 practice Q

Optical media capacity matching

How to spot it:

'Capacity of a CD, DVD or Blu-ray' or 'blue-violet laser'.

Method

  1. Recall the row: CD 700 MB, DVD 4.7 GB, Blu-ray 25 GB.

  2. Dual layer: DVD about 8.5 GB, Blu-ray about 50 GB.

  3. For ordering, use CD, then DVD, then Blu-ray.

  4. Blue-violet laser means Blu-ray.

Why it works:

Shorter laser wavelength packs data tighter.

Try this

A single-layer DVD stores about...

Show solution

  1. The 700 MB figure is a CD.

  2. 25 GB is Blu-ray.

Answer

4.7 GB

Type 2very common5 practice Q

HDD vs SSD vs flash

How to spot it:

'No moving parts', 'faster', 'spinning platters' or 'shock-resistant'.

Method

  1. HDD: spinning magnetic platters and heads.

  2. SSD and pen drive: flash chips, no moving parts.

  3. SSD is faster, silent and shock-resistant.

  4. Flash keeps data without power.

Why it works:

Moving parts make an HDD slower than an SSD.

Try this

Why is an SSD faster than an HDD?

Show solution

  1. An SSD has no platters to spin.

  2. It has no heads to seek.

Answer

It has no mechanical parts

Type 3common3 practice Q

Backup and access methods

How to spot it:

'Sequential', 'random access', 'archival' or 'cloud backup'.

Method

  1. Archival backup: magnetic tape, cheapest per TB.

  2. Sequential means read in order (tape).

  3. Random means jump to any address (disk, SSD).

  4. Copies on internet servers are cloud backup.

Why it works:

Access method decides how fast a medium finds data.

Try this

Which storage medium uses sequential access?

Show solution

  1. The drive must wind past earlier data.

  2. Disks and SSDs jump directly instead.

Answer

Magnetic tape

Type 4common2 practice Q

Media odd-one-out and matching

How to spot it:

'Which is NOT an optical medium' or 'match the medium to its type'.

Method

  1. Optical: CD, DVD, Blu-ray.

  2. Magnetic: HDD, floppy, tape.

  3. Flash: SSD, pen drive, memory card.

  4. Find the option that breaks the family.

Why it works:

Each medium belongs to exactly one family.

Try this

Which of the following is NOT an optical storage medium? CD, DVD, Hard disk, Blu-ray

Show solution

  1. CD, DVD and Blu-ray are read by lasers.

  2. A hard disk is magnetic.

Answer

Hard disk

Type 5occasional

Primary vs secondary storage

How to spot it:

'Secondary storage', 'permanent', 'primary memory' or 'formatting'.

Method

  1. Primary memory: RAM and cache. Fast and small.

  2. Secondary storage: disks, tape, flash. Large and permanent.

  3. Formatting prepares the file system only.

  4. Use everyday sizes to reject silly options.

Why it works:

Primary is for speed. Secondary is for keeping data.

Try this

Which of these is secondary storage? RAM, Cache, SSD, Registers

Show solution

  1. RAM, cache and registers are primary memory.

  2. An SSD keeps data without power.

Answer

SSD

10

Shortcuts that save time

⚡ Optical capacity ladder

CD is 700 MB. DVD is 4.7 GB. Blu-ray is 25 GB (single layer). Say it as '700, 4.7, 25'.

Example

What is the capacity of a single-layer DVD?

Show solution

The DVD is the middle rung of the ladder.

Answer

4.7 GB

⚡ SSD or HDD in one line

SSD has no moving parts. It is silent, shock-proof, faster and costlier per GB. HDD has spinning platters and gives cheap bulk storage.

Example

Which storage has no moving parts?

Show solution

Flash chips have nothing that spins.

Answer

SSD (solid state drive)

11

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Giving 4.7 GB as the capacity of a CD.

A CD holds 700 MB. 4.7 GB is a DVD.

Mistake 02

Calling a hard disk an optical medium.

A hard disk is magnetic. CD, DVD and Blu-ray are optical.

Mistake 03

Saying an SSD has spinning platters.

An SSD uses flash chips. HDDs have the platters.

Mistake 04

Thinking tape gives random access.

Tape is sequential. Disks and SSDs are random access.

Mistake 05

Thinking formatting adds capacity.

Formatting only prepares the file system.

12

Quick revision

Read this the night before the exam.

  • Magnetic: HDD, tape. Optical: CD, DVD, Blu-ray. Flash: SSD, pen drive, memory card.

  • CD 700 MB, DVD 4.7 GB (dual 8.5 GB), Blu-ray 25 GB (dual 50 GB).

  • Blu-ray uses a blue-violet laser.

  • SSD has no moving parts and is faster and costlier. HDD is cheaper per GB.

  • Tape is sequential and used for archival backup. Disk and SSD are random access.

  • Cloud backup keeps copies on internet servers. A floppy held 1.44 MB.

13

Practice: 16 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 4 min · wrong answers go to your mistake notebook automatically.

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