Learning Objectives
- Describe how a hard disk drive stores and retrieves data magnetically.
- Explain the roles of platters, tracks, sectors, read/write heads and electromagnets.
- Describe the sequence used to read and write data on a hard disk.
- Evaluate the advantages and limitations of magnetic storage.
- Select a hard disk appropriately for high-capacity storage.
Key Terms
- Magnetic storage
- Storage in which binary data is represented by magnetic states on a surface.
- Hard disk drive (HDD)
- A secondary-storage device containing rotating magnetic platters and moving read/write heads.
- Platter
- A rigid disk coated with magnetic material.
- Track
- A concentric circular path on a platter used to organise data.
- Sector
- A subdivision of a track used as an addressable block of storage.
- Read/write head
- A component positioned close to a platter that detects or changes magnetic states.
- Electromagnet
- A magnet produced by electric current, used by a write head to alter magnetic orientation.
- Spindle
- The central mechanism that rotates the platters.
- Seek time
- The time taken to move a read/write head to the required track.
- Rotational delay
- The time spent waiting for the required sector to rotate under the head.
- Fragmentation
- A condition in which parts of a file are stored in non-adjacent locations.
Physical Structure Of A Hard Disk Drive
A hard disk drive contains one or more rigid platters coated with magnetic material. The platters rotate rapidly on a spindle. Each surface is organised into concentric tracks, and each track is divided into sectors. These divisions allow the controller to identify where blocks of data are stored.
A read/write head is positioned extremely close to each recording surface. An actuator arm moves the head across the radius of the platter to the required track. The head does not normally touch the surface during operation, because contact could damage the magnetic coating and stored data.
The drive controller receives requests from the computer, determines the required physical locations and coordinates head movement, platter rotation and data transfer.
Writing Data Magnetically
To write data, the drive moves the head to the required track and waits for the correct sector to rotate underneath. Electric current through the write head creates a magnetic field. The field changes the magnetic orientation of tiny regions on the platter surface.
Different magnetic states represent binary values. The exact physical encoding is more complex than one visible magnet for every bit, but the required principle is that electromagnets change magnetic patterns to store binary data.
The drive may verify a write and maintain error-control information. The operating system and file system decide which logical blocks belong to a file, while the hardware performs the physical magnetic recording.
Reading Data
To read data, the head is moved to the track containing the requested sector. As magnetic regions pass under the head, changes in magnetic field induce an electrical signal. The drive electronics interpret the signal as binary data and send the resulting block to the computer.
Access time includes seek time, rotational delay and transfer time. Because the head and platter move mechanically, random access is slower than in solid-state storage. Sequential access to nearby blocks can be faster because less head movement is required.
A hard disk can be damaged by shock while operating. Moving parts also generate noise, heat and power consumption. These factors matter when comparing it with an SSD.
Advantages, Limitations And Applications
Hard disk drives offer high capacity at a relatively low cost per gigabyte. They are suitable for desktop storage, backups, servers and large collections of media where cost and capacity are more important than very low access time.
Limitations include mechanical delay, sensitivity to impact, greater power use and wear of moving components. They are usually heavier and noisier than solid-state alternatives. Performance can also be affected when many small files are scattered across the platter.
A balanced evaluation should connect the application to both sides. An HDD may be a good choice for economical bulk storage but a weaker choice for a frequently moved device requiring fast startup and shock resistance.
Hard Disk Components
| Component | Function |
|---|---|
| Platter | Provides the magnetic recording surface. |
| Track | Organises data in concentric circular paths. |
| Sector | Divides a track into addressable storage blocks. |
| Read/write head | Detects magnetic patterns and changes them when writing. |
| Actuator arm | Moves the head to the required track. |
| Spindle | Rotates the platters. |
| Controller | Coordinates physical access and transfers data to the computer. |
Magnetic Storage Evaluation
| Advantage | Explanation | Possible Limitation |
|---|---|---|
| High capacity | Large amounts of data can be stored on multiple platter surfaces. | The device is physically larger than many flash devices. |
| Low cost per gigabyte | Mature magnetic technology is economical for bulk storage. | Mechanical access is slower than solid-state access. |
| Rewritable | Magnetic regions can be changed many times. | Moving parts may wear or fail. |
| Suitable for large files | Good sustained transfer for nearby blocks. | Random access includes seek and rotational delay. |
| Non-volatile | Data remains without power. | Shock can damage an operating drive. |
Worked Examples
Describing A Disk Read
Question: Describe how an HDD reads a block stored in a particular sector.
- The actuator moves the read/write head to the correct track.
- The platter rotates until the required sector is under the head.
- The head detects changes in magnetic fields on the surface.
- Electronics decode the signal into binary data.
- The block is transferred to the computer.
Answer: Mechanical positioning locates the sector, then the head senses its magnetic pattern and the controller returns the decoded data.
Choosing Bulk Storage
Question: A media archive needs 12 TB of inexpensive internal storage and is not moved frequently. Explain why HDDs may be suitable.
- The archive requires very high capacity.
- HDDs offer a low cost per gigabyte.
- The system is stationary, reducing shock risk.
- The archive prioritises bulk capacity more than the fastest random access.
Answer: HDDs are suitable because they provide economical multi-terabyte storage and the stationary use reduces the importance of impact resistance.
Examination Guidance
- Use all four required terms when describing structure: platters, tracks, sectors and read/write heads.
- Mention electromagnets or magnetic fields when explaining writing.
- For reading, explain head positioning, rotation and detection of magnetic changes.
- Link slower random access to mechanical movement.
- Do not present low cost per gigabyte as proof that an HDD is best for every device.
Common Mistakes
- Saying an HDD reads data using a laser.
- Confusing tracks with sectors.
- Saying the head normally scrapes across the platter surface.
- Describing magnetic storage as volatile.
- Ignoring shock risk and mechanical delay when comparing with an SSD.
Knowledge Check
1. How are tracks arranged on a platter?
2. What is a sector?
3. How is data written magnetically?
4. Why is random access slower than on an SSD?
5. What is a major advantage of an HDD?
