GCSE · Computer Science · AQA · Spec 8525

Secondary storage

Switch a computer off and its RAM forgets everything. Switch it back on and your photos, games and operating system are all still there. So what remembered them?

Secondary storage · The three types

Sort the storage

Which type of secondary storage is each one?

Still to sort

Solid state (0)

Stores data electronically, in chips. No moving parts.

Where the line is: No laser and no magnet: the bit is an electric charge held (or not) in a tiny cell on a chip.

Optical (0)

A reflective disc that a laser reads and writes.

Where the line is: A hard disk drive spins too, but its platters are magnetic. Optical is the one where the bit is read with light: does the laser bounce back or scatter?

Magnetic (0)

Magnetised spots on fast-spinning platters.

Where the line is: Not read by a laser. A read/write head on a moving arm senses which way each magnetised spot points.

7 of 7 still to sort.

Pick a piece of storage, then choose its type. Each answer tells you how that type physically holds a 1 or a 0.

Watch out: A CD and a hard disk drive both sound like discs, but they are different types. Ask how the bit is held: laser light bouncing back or scattering off pits and lands is optical; magnetised spots on a spinning platter is magnetic.

Why RAM isn't enough

Main memory (RAM)vsSecondary storage

Start with the row that matters most: what happens when you switch off.

Focus

When the power goes off

Main memory (RAM)

Loses its data and programs

Secondary storage

Keeps them: it is non-volatile

The insight

This is the whole reason secondary storage exists. Your files and programs have to survive being switched off.

What it holds

Main memory (RAM)

Data and programs only while they are in use

Secondary storage

All files, folders and programs, including the operating system, for the long term

Cost per gigabyte

Main memory (RAM)

More expensive

Secondary storage

Cheaper

Capacity

Main memory (RAM)

Less

Secondary storage

More

Predict, then check

Two words that sound alike and mean different things.

You take a photo, and it is saved onto the memory card inside your camera. Which is the storage media and which is the storage device?

Comparing the three types

Build each type's profile

Tick every property that is true of each type. Then check your grid.

Solid state
Magnetic
Optical

Exam line: When you choose a type, name the characteristic that matters for the job and say how the type compares on it.

Choosing storage

What makes a computer load faster?

You're choosing the storage for a new computer. You want programs and files to load as quickly as possible.

Which is closest to what you think right now?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Three completely different ways to remember a 1 or a 0 with the power off.

What you need to know

  • RAM (main memory) only holds data and programs while they are in use. Programs and files must be kept when they are not being used and when the computer has no power, so a computer needs secondary storage.
  • Secondary storage is non-volatile: it keeps data when the computer is turned off. It stores all files, folders and programs long term, including the operating system.
  • Compared with main memory, secondary storage is cheaper per gigabyte and has more capacity. It is robust and reliable, and can be portable.
  • Storage media is the physical material data is stored on (for example a hard disk drive, SSD, USB flash drive, memory card or CD). A storage device is the hardware that reads from and writes to the media (for example a computer, a mobile phone or a camera).
  • Solid state storage (SSDs, memory cards, USB flash drives) has no moving parts. Its chips contain billions of tiny cells, and each cell either holds an electric charge or not, representing 1 or 0.
  • Optical storage (CDs, DVDs, Blu-ray discs) uses a laser. Data is burned onto a reflective disc as tiny pits and lands, and a sensor detects whether the laser light bounces back or scatters to read each 0 or 1.
  • Magnetic storage (hard disk drives) stores data as magnetised spots on fast-spinning platters. A read/write head on a moving arm flips the direction of a spot to write (one direction means 0, the other 1) and detects which way spots point to read.
  • The three types are compared by capacity, speed, portability, durability, reliability and cost.
  • Speed depends on the type of drive, not its capacity. Upgrading from an HDD to an SSD improves speed; a bigger drive of the same type does not.

The big picture

RAM only holds data and programs while they are in use, and loses them when the power goes. So a computer needs secondary storage: it is non-volatile, cheaper per gigabyte and holds more. There are three types, each holding a 1 or 0 with a different physical trick: a charged or uncharged cell (solid state), laser light bouncing back or scattering off pits and lands (optical), or a spot magnetised one way or the other (magnetic). The trick explains the trade-offs. Solid state is fast and durable but costs more; magnetic holds lots cheaply; optical is very cheap and easy to move, but slower and can be scratched.

Key points

1Every type keeps the same 1s and 0s with the power off. They differ in the trick: charge, light or magnetism.
2Solid state: fast, durable, silent and needs less power, but more expensive per gigabyte.
3Magnetic: high capacity, moderate speed and lower cost per gigabyte, but its moving parts can be damaged.
4Optical: very low cost per gigabyte and easily moved between devices, but lower capacity, slower, and can be scratched or degrade over time.
5Media is what the data is on; the device is what reads and writes it.

Worked example

Problem

A delivery driver's handheld scanner gets dropped and bumped all day. It has to keep the day's delivery records, even when its battery runs out. Which type of secondary storage should it use, and why is a hard disk drive a poor choice?

⚠ Watch out

Putting a hard disk drive in the same group as a CD because both sound like discs. The type depends on how the bit is held: a laser reading pits and lands is optical; magnetised spots on spinning platters are magnetic.

🧠

Memory hook

Charge, light, magnet. A solid state cell holds a charge or doesn't. An optical laser bounces back or scatters. A magnetic spot points one way or the other. Remember the trick, and the trade-offs follow.

✓

Check yourself

Cover the page. For solid state, optical and magnetic storage, say how each one holds a 1 or a 0, then give one strength and one weakness of each.

Flashcards

(13)
Why can't a computer rely on RAM alone to keep your files?
RAM only holds data and programs while they are in use, and loses them when the power goes. Files and programs must be kept when not in use.
What does it mean that secondary storage is non-volatile?
It keeps data stored even when the computer is turned off, so it is used for long-term storage of files, folders and programs, including the operating system.
How does secondary storage compare with main memory on cost and capacity?
Secondary storage is cheaper per gigabyte and has more capacity.
Storage media vs storage device: what's the difference?
Media is the physical material the data is stored on (e.g. a memory card). The device is the hardware that reads from and writes to it (e.g. a camera).
Name the three types of secondary storage.
Solid state, optical and magnetic.
How does solid state storage hold a 1 or a 0?
Its chips contain billions of tiny cells. Each cell either holds an electric charge or not.
How is data stored on and read from an optical disc?
It is burned onto a reflective disc as pits and lands. A laser shines on it and a sensor detects whether the light bounces back or scatters.
How does a hard disk drive write and read a bit?
A read/write head on a moving arm flips the direction of a magnetised spot on a spinning platter to write (one direction 0, the other 1), and detects which way it points to read.
Durability vs reliability
Durability: withstanding shock, impact or environmental factors. Reliability: storing and retrieving data accurately without failure over time.
Solid state: main strengths and main drawback?
Fast read/write, durable (no moving parts), needs less power, silent. Drawback: more expensive per gigabyte.
Magnetic: main strengths and main drawback?
High capacity and lower cost per gigabyte. Drawback: moving parts mean it can be damaged.
Optical: main strengths and main drawbacks?
Very low cost per gigabyte and easily moved between devices. Drawbacks: lower capacity, slower, can be scratched or degrade over time.
Will a bigger hard disk drive make a computer load programs faster?
No. Speed depends on drive type, not capacity. Swapping an HDD for an SSD improves speed.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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