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Primary vs secondary storage
Primary memory (RAM, cache) is fast but small and (for RAM) volatile. Secondary storage is large, permanent and cheaper per GB — everything below is secondary.
Hardware, Memory & Number Systems
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Storage and backup media
🔒 Log in to track⏱ 5 min read🧩 4 question types🎯 16 practice Q
The idea in one minute
| Medium | Technology | Capacity (typical) |
|---|---|---|
| Hard Disk Drive (HDD) | Magnetic platters, spinning | 500 GB - 20 TB |
| SSD / NVMe | Flash chips, no moving parts | 128 GB - 8 TB, much faster than HDD |
| CD-ROM | Optical, 1 side | ~700 MB |
| DVD (single layer) | Optical | 4.7 GB (dual-layer 8.5 GB) |
| Blu-ray (single layer) | Blue-violet laser | 25 GB (dual 50 GB) |
| Pen drive / SD card | Flash | GBs - TBs |
| Magnetic tape (LTO) | Sequential magnetic | TBs per cartridge; archival backup |
| Cloud storage | Remote servers | TB plans; off-site backup |
Optical discs store bits as pits/lands read by laser; the shorter the wavelength (IR -> red -> blue-violet), the denser the storage (CD -> DVD -> Blu-ray). Backup rule of thumb: keep at least one copy off-site and on a different medium than the live data. NAS (Network Attached Storage) gives shared disk over the LAN.
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The three technologies
| Family | How it stores | Examples |
|---|---|---|
| Magnetic | Magnetised spots on spinning platters or tape | HDD (hard disk), old floppy, magnetic tape (LTO) |
| Optical | Tiny pits read by a laser | CD, DVD, Blu-ray |
| Flash (solid-state) | Floating-gate memory chips, no moving parts | SSD, pen drive, memory card, smartphone storage |
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Capacities to memorise (the exam's favourite row)
| Disc | Capacity (single layer) | Laser |
|---|---|---|
| CD | 700 MB | infrared/red |
| DVD | 4.7 GB (dual-layer ≈ 8.5 GB) | red |
| Blu-ray | 25 GB (dual-layer ≈ 50 GB) | blue-violet (shorter wavelength → tighter pits → more data) |
Ladder for one-mark questions: CD (0.7 GB) < DVD (4.7 GB) < Blu-ray (25 GB).
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HDD vs SSD (asked almost every year)
| HDD | SSD | |
|---|---|---|
| Parts | Spinning magnetic platters + moving heads | Flash chips, no moving parts |
| Speed | Slower (heads must seek) | Much faster |
| Shock / noise | Sensitive, some noise | Shock-resistant, silent |
| Cost per GB | Cheaper | Costlier |
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Access methods
- Random (direct) access — jump straight to any address: RAM, HDD, SSD, disc with a file system.
- Sequential access — read through in order, like a cassette: magnetic tape. That is why tape is cheap and perfect for archival backups, not for daily use.
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Backup ideas
- Cloud storage/backup — your files copied to internet servers (anywhere access).
- Tape libraries — banks and research labs still archive petabytes on tape.
- The 3-2-1 habit: three copies, two media, one off-site — a good phrase for one-liners.
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Everyday capacity anchors (for rejecting silly options)
- One MP3 song ≈ 4–8 MB; a 90-minute movie ≈ 1–4 GB; a phone ≈ 64–256 GB; a laptop SSD ≈ 512 GB–1 TB; a data-centre shelf ≈ many TB.
- So 'a CD holds 4.7 GB' should feel wrong instantly — that is DVD territory.
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One-line differentiators exams quote
- Floppy disk — the 1.44 MB magnetic ancestor, long obsolete but still asked for its capacity.
- Memory card — flash storage for cameras and phones (SD, microSD).
- Blu-ray's name comes from its blue-violet laser, not from any brand.
- Formatting prepares a disk's file system; it does not add capacity.
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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/Blu-ray?', 'arrange by capacity', 'dual-layer DVD holds', 'which disc uses a blue-violet laser'.
Method
One row: CD = 700 MB, DVD = 4.7 GB (dual ≈ 8.5 GB), Blu-ray = 25 GB (dual ≈ 50 GB).
Ordering questions run CD < DVD < Blu-ray.
The blue-violet (shorter-wavelength) laser is Blu-ray's identifying fact.
Try this
A single-layer DVD stores about —
Show solutionHide solution
4.7 GB (a dual-layer DVD ≈ 8.5 GB). The 700 MB figure belongs to a CD; 25 GB to a single-layer Blu-ray.
Type 2very common5 practice Q
HDD vs SSD vs flash
How to spot it:
'Which storage has no moving parts?', 'why is an SSD faster?', 'a pen drive uses which memory?', 'HDD stores data on…'.
Method
HDD = spinning magnetic platters + heads; SSD/pen drive = flash chips, no moving parts.
SSD's speed comes from zero mechanical seek time; it is also silent and shock-resistant.
Flash is non-volatile — data stays without power, unlike RAM.
Try this
Why is an SSD faster than an HDD?
Show solutionHide solution
The SSD has no mechanical parts — no platters to spin up, no heads to seek — so access takes microseconds instead of milliseconds.
Type 3common3 practice Q
Backup and access methods
How to spot it:
'Which medium for archival backup?', 'sequential access example', 'storing backups on internet servers is called'.
Method
Archival = magnetic tape (LTO): cheapest per TB, sequential access.
Sequential = read in order (tape, cassette); random/direct = jump to any address (disk, SSD).
Copies on internet servers = cloud storage/backup.
Try this
Which storage medium uses sequential access, reading data strictly in order?
Show solutionHide solution
Magnetic tape — like a cassette, the drive must wind past everything before the needed block. Disks and SSDs give random (direct) access.
Type 4common2 practice Q
Media odd-one-out and matching
How to spot it:
'Which is NOT an optical medium?', 'match disc to capacity', 'which is a magnetic storage device?'.
Method
Optical family = CD, DVD, Blu-ray (laser-read). Anything else (HDD, pen drive, tape) breaks the family.
Magnetic family = HDD, floppy, tape. Flash family = SSD, pen drive, memory card.
For matching, pair by the capacity row first, then by technology.
Try this
Which of the following is NOT an optical storage medium?
Show solutionHide solution
Hard disk — it is magnetic. CD, DVD and Blu-ray are all read by lasers, which defines optical storage.
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Shortcuts that save time
⚡ Optical capacity ladder
CD 700 MB -> DVD 4.7 GB -> Blu-ray 25 GB (single layer). Roughly: CD 0.7, DVD 4.7, BR 25 - '700, 4.7, 25'.
Example
Storage capacity of a single-layer DVD?
Show solutionHide solution
4.7 GB.
⚡ SSD vs HDD one-liner
SSD = no moving parts (flash), silent, shock-proof, faster, costlier per GB. HDD = spinning platters, cheap bulk storage. 'No moving parts' in a question = SSD.
Example
Which storage has no moving parts?
Show solutionHide solution
Solid State Drive (flash memory).
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Mistakes to avoid
Where most students lose marks on this subtopic.
Mistake 01
Quoting 700 MB for a DVD - that is a CD; a single-layer DVD is 4.7 GB.
Mistake 02
Saying Blu-ray uses a red laser - Blu-ray uses the shorter blue-violet laser.
Mistake 03
Believing tape is obsolete - LTO tape is still standard for archival backups.
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Quick revision
Read this the night before the exam.
Magnetic = HDD, tape; optical = CD/DVD/Blu-ray (laser-read pits); flash = SSD, pen drive.
CD 700 MB → DVD 4.7 GB → Blu-ray 25 GB (blue-violet laser); DVD dual ≈ 8.5 GB.
SSD: no moving parts, faster, costlier, shock-proof. HDD: magnetic platters, cheaper per GB.
Tape = sequential access, archival backup; disk/SSD = random access.
Cloud backup = copies held on internet servers.
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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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