3.1.4 Secondary Storage Technologies
Secondary storage keeps programs and data available after electrical power is removed. Different devices achieve this in very different ways: a hard disk changes magnetic patterns, an optical drive uses laser light, and solid-state storage holds data in electronic memory cells.
This lesson focuses on the main operating stages of the three technologies and on choosing a suitable device for a particular situation.
By the end of this section, you should be able to:
- Explain why computer systems require non-volatile secondary storage.
- Describe how a magnetic hard disk writes, locates, and reads stored data.
- Explain how an optical disc reader/writer uses a laser and a reflective recording layer.
- Describe how flash memory stores, reads, writes, and erases data.
- Use terms such as platter, read-write head, track, sector, pit, land, page, block, and controller accurately.
- Compare storage technologies by linking their physical operation to speed, durability, portability, capacity, and cost.
- Justify an appropriate storage choice for a stated use.
Why secondary storage is needed
Primary memory supports programs that are currently running, but most of it does not retain its contents when power is lost. A computer therefore needs a separate place for operating-system files, applications, user documents, media, and backups.
A storage medium is the material or electronic structure on which the data is held. A storage device contains the mechanisms and control electronics that read from or write to that medium.
| Technology | Where the data is held | How the device accesses it |
|---|---|---|
| Magnetic hard disk | Magnetised areas on rotating platters | A moving read-write head detects or changes magnetic states |
| Optical disc | A patterned or changeable recording layer on a disc | A laser and detector read differences in reflected light |
| Solid-state storage | Electronic charge states in flash memory cells | A controller addresses memory electronically |
Common mistake
Secondary does not mean unimportant. It describes the storage role, not its value to the system. Without secondary storage, files and installed software would not remain available after shutdown.
Magnetic hard disks
A hard disk drive contains one or more rigid platters coated with magnetic material. The platters rotate around a spindle. An actuator moves a read-write head to the required region while the platter spins beneath it.
Writing data
- The controller identifies the sector where the data will be stored.
- The actuator positions the read-write head over the correct track.
- As the platter rotates, the head changes tiny magnetic regions on its surface.
- Different magnetic states form the stored binary pattern.
Reading data
- The head is moved to the required track.
- The requested sector rotates beneath the head.
- The head senses changes in magnetisation.
- The drive electronics convert the detected pattern into binary data.
| Term | Role |
|---|---|
| Platter | A rotating disk surface coated with magnetic material. |
| Spindle | Rotates the platters at a controlled speed. |
| Actuator arm | Moves the read-write head across the platter. |
| Read-write head | Changes magnetic states when writing and detects them when reading. |
| Track | A circular storage path on a platter surface. |
| Sector | A numbered part of a track used as an addressable storage unit. |
Hard disks provide direct access: the drive can move to a requested sector rather than reading every earlier sector first. Access still takes time because mechanical movement and platter rotation are required.
Exam tip
A strong description links the sequence together: sector selected → head positioned → platter rotates → magnetic pattern written or detected → binary data produced.
Common mistake
Do not say that the read-write head touches the platter. It operates extremely close to the surface; physical contact could damage the disk.
Interactive: Magnetic hard disk structure
Use the explorer to connect each physical part of a hard disk with the reading and writing process.
Optical disc readers and writers
Optical storage includes technologies such as CD, DVD, and Blu-ray Disc. The disc contains a single spiral data path. During use, the disc rotates while an optical assembly keeps a laser focused on the required part of the track.
Reading an optical disc
- The drive positions the laser over the required part of the spiral track.
- The laser shines onto the disc surface.
- Pits and lands, or differently recorded regions, return different light patterns.
- A detector measures the reflected light.
- The drive electronics convert the detected changes into binary data.
Writing to a recordable or rewritable disc
A writing laser uses more energy than the reading laser. It alters the recording layer to create regions with different optical properties. On a rewritable disc, controlled heating can return the material to another state so that the area can be used again.
| Feature | Purpose |
|---|---|
| Spiral track | Provides one continuous route for the stored data. |
| Pit and land | Create differences or transitions in reflected light that the drive can detect. |
| Laser | Reads the surface and, in a writer, changes the recording layer. |
| Detector | Measures reflected light and sends an electrical signal for decoding. |
| Shorter wavelength | Allows a smaller focused spot, so recorded features can be placed closer together. |
Exam tip
For capacity questions, explain the causal chain: shorter wavelength → smaller focused spot → denser recording → greater capacity.
Common mistake
Do not describe an optical disc as a set of separate concentric tracks. Its data path is one continuous spiral.
Interactive: Optical disc reading
Follow the path from the laser to the detector and then to the reconstructed bit pattern.
Solid-state and flash storage
Solid-state storage uses semiconductor memory rather than a rotating disc. Data is held in flash memory cells, and a controller manages the location, movement, and protection of the stored data.
Reading, writing, and erasing
- The controller translates a request into the physical flash locations that contain the data.
- Data can be read from a page.
- New data is written electronically into available pages.
- Before previously used cells can be written again, a larger block must be erased.
- The controller reorganises data and spreads writes across the memory to reduce uneven wear.
| Term | Meaning |
|---|---|
| Flash memory cell | An electronic structure whose charge state represents stored information. |
| Page | A group of cells handled together during read or write operations. |
| Block | A larger group of pages erased as one unit. |
| Controller | Maps logical requests to physical locations and manages reading, writing, erasing, and wear. |
With no head movement or platter rotation, solid-state storage usually provides quick access and tolerates movement better than a hard disk. However, flash cells support only a limited number of program-and-erase cycles, so gradual wear still occurs.
Common mistake
“No moving parts” does not mean “cannot fail”. Solid-state devices avoid mechanical wear, but flash cells still have a finite write endurance.
Interactive: Solid-state flash structure
Use the explorer to see how cells, pages, blocks, and the controller work together.
Choosing an appropriate technology
A suitable choice depends on the context. A good justification should connect a requirement to a physical property and then to its practical consequence.
| Situation | Reasonable choice | Justification |
|---|---|---|
| A low-cost desktop archive storing many large video files | Magnetic hard disk | High capacity can be provided economically when portability and shock resistance are not priorities. |
| A field laptop used while travelling over rough ground | Solid-state drive | No moving head or platter makes it less vulnerable to movement, and access is generally fast. |
| Distributing a fixed collection of training videos to many offline sites | Read-only optical disc | The content is portable, can be duplicated, and is not intended to change during normal use. |
| A small removable device used to transfer project files | USB flash storage | It is compact, rewritable, and has no moving mechanical parts. |
Exam tip
Avoid unsupported claims such as “SSD is best”. Write: requirement → relevant property → consequence for the user.
Exam support
Describing operation
Use a clear sequence with active verbs: rotate, position, magnetise, focus, reflect, detect, decode, address, write, erase.
Comparing technologies
State a technical difference and then explain its effect. For example: “A hard disk has moving parts, so locating data involves mechanical delay.”
Common mistakes and misconceptions
- Listing advantages without explaining how the technology creates them.
- Calling a hard disk “random access” without explaining sector selection.
- Describing an optical disc as magnetic storage.
- Forgetting that an optical writer changes the recording layer.
- Saying flash memory erases one cell at a time; erase operations use blocks.
- Claiming that solid-state storage has unlimited lifespan.
Practice
Knowledge and explanation
- Explain why secondary storage is needed even when a computer has RAM.
- Describe how a hard disk writes a binary pattern to a chosen sector.
- Explain why mechanical movement affects hard-disk access time.
- Describe how an optical disc reader converts reflected light into data.
- Explain how a rewritable optical drive changes the disc surface.
- Why can a shorter laser wavelength increase optical-disc capacity?
- Explain the difference between a page and a block in flash memory.
- Why must a flash block sometimes be erased before new data is written?
Original application task
A conservation team collects high-resolution photographs in a remote forest. The storage device will be carried between sites and may experience vibration.
- Choose a suitable storage technology.
- Explain two technical reasons for your choice.
- Identify one limitation that the team should still consider.
Compare and justify
A media department wants one device for inexpensive long-term local storage and another for fast editing of current projects. Recommend a technology for each purpose and justify both choices.
Review
| Technology | Core operating idea | Key terminology |
|---|---|---|
| Magnetic hard disk | A moving head reads and changes magnetic patterns on rotating platters. | Platter, spindle, actuator, read-write head, track, sector |
| Optical disc | A laser reads reflection changes and can alter a recording layer when writing. | Spiral track, pit, land, laser, detector, wavelength |
| Solid-state storage | A controller manages electronic charge states in flash memory cells. | Cell, page, block, controller, erase-before-write, wear |