Types of secondary storage
Types of Secondary Storage
Secondary storage provides non-volatile, permanent data retention. The two main categories at GCSE level are magnetic storage (hard disk drives) and solid-state storage (solid-state drives). They store data using completely different physical principles, which directly explains their differences in speed, durability, cost, and appropriate use cases.
Magnetic and Solid-State Storage
Hard Disk Drive (HDD) - Magnetic Storage
A hard disk drive (HDD) stores data magnetically on one or more rigid, spinning discs called platters. Each platter is coated with a magnetic material that can be magnetised in two orientations to represent binary 0s and 1s. A read/write head on a mechanical arm moves across the platter surface to read data (by detecting magnetic fields) or write data (by changing the magnetic orientation of tiny regions on the surface).
The read/write head moves across spinning platters to access data stored magnetically on concentric tracks.
HDDs are mechanical devices - the platters spin at high speed (typically 5,400-7,200 revolutions per minute) and the arm physically moves to position the head over the correct track. To read any piece of data, the drive must:
- Seek: move the arm to the correct track (seek time)
- Wait: allow the platter to rotate until the correct sector passes under the head (rotational latency)
- Transfer: read or write the data
This physical movement is what makes HDDs slower than SSDs - no matter how fast the motor spins, mechanical positioning takes significantly longer than electronic switching.
Solid-State Drive (SSD) - Solid-State Storage
A solid-state drive (SSD) stores data using electrical circuits - specifically, flash memory chips that can retain their state without power. Unlike an HDD, an SSD has no moving mechanical parts whatsoever. Data is stored as electrical charges in memory cells arranged in a grid. Writing data means setting the electrical state of cells; reading data means detecting those states.
SSDs use electrical circuits and flash memory chips. A controller manages reading and writing across the chips.
Because there are no moving parts, data access is almost instantaneous - the controller simply addresses the correct memory cells electronically. There is no seek time and no rotational latency. This makes SSDs significantly faster than HDDs for both reading and writing. SSDs are also silent, generate less heat, and are far more resistant to physical shocks than HDDs.
The electrical circuits in flash memory retain their state without power, making SSDs non-volatile just like HDDs. Students do not need to know the specific circuit details (such as the use of NAND gates) at GCSE level - the key point is that electrical storage enables persistent, power-free data retention.
Advantages and Disadvantages
| Criterion | HDD (magnetic) | SSD (solid state) |
|---|---|---|
| Speed | Slower - mechanical seek and rotation adds latency. Typical read/write: 80-160 MB/s | Much faster - purely electronic access. Typical read/write: 500-7000 MB/s |
| Durability | Fragile - moving parts vulnerable to shock, drops, and vibration. Can be damaged by physical impacts | Highly durable - no moving parts. Resistant to drops, vibration, and physical shock |
| Noise | Audible - spinning platters and moving arm produce noise, especially when seeking | Silent - no mechanical components |
| Power consumption | Higher - motor must spin platters continuously | Lower - reduces battery drain in laptops and mobile devices |
| Capacity | Very high capacity at low cost. Consumer HDDs commonly offer 1-20 TB | Large capacities available but cost per gigabyte remains higher than HDD |
| Cost per GB | Cheaper per gigabyte | More expensive per gigabyte |
| Typical use | Large-capacity storage where cost matters: desktop PCs, NAS (network attached storage), archival storage | Speed-critical applications: laptop boot drives, OS drives, gaming, professional workstations |
Key Takeaways
- HDDs store data magnetically on spinning platters. A mechanical read/write head moves to access data. They are slower, less durable, but cheaper per gigabyte and available in very large capacities.
- SSDs store data using electrical circuits (flash memory chips). There are no moving parts. They are faster, more durable, silent, and energy-efficient, but more expensive per gigabyte.
- HDDs are suited to large-capacity, cost-sensitive storage. SSDs are suited to speed-critical applications and portable devices.
- Both types are non-volatile - they retain data without power. SSDs achieve this through electrical circuits; HDDs achieve it through stable magnetic states on the platter surface.