Secondary storage
Why secondary storage is needed
Secondary storage keeps files and programs when the computer is switched off. It has much greater capacity than primary memory and is non-volatile. The three types are optical, magnetic and solid state storage.
A school might keep a long-term backup on an external hard disk, distribute a video on a Blu-ray disc, or use a portable SSD to move large project files.
Key terms
Comparing secondary storage
Compare the capacity, speed, portability, durability, reliability and cost for the job. These are general comparisons: a particular product can differ.
| Characteristic | Optical (CD/DVD/Blu-ray) | Magnetic (HDD) | Solid state (SSD/flash) |
|---|---|---|---|
| Capacity | Usually lower; Blu-ray holds more than CD or DVD | Often high for its price | High, but usually costs more per byte than HDD |
| Speed | Slowest to access and transfer | Faster than optical; slower than SSD | Fast access and transfer |
| Portability | Discs are light and easy to post | External drives can be carried, but are bulkier | Small and light; memory cards and USB drives are very portable |
| Durability | Disc surface can be scratched | Moving parts can be damaged by knocks | No moving parts, so handles knocks better |
| Reliability | Scratches or ageing can make data unreadable | Mechanical parts can wear or fail | No moving parts, but flash cells have a limited number of writes |
| Cost | Discs are cheap per item, but capacity is limited | Low cost per byte for large storage | Higher cost per byte than HDD in many typical comparisons |
Choose for the situation: optical is suitable for distributing a read-only film; a magnetic HDD suits a large, low-cost desktop backup; solid state suits a laptop drive that needs to be fast and withstand everyday movement. For any one purchase, compare current products rather than assuming these broad patterns guarantee a specific device's performance.