3.1.2 Secondary storage
Why Secondary Storage Is Needed
RAM is volatile — data is lost when power is removed. Secondary storage provides non-volatile, long-term storage for programs and data that must persist between sessions. Secondary storage is much larger than RAM but slower to access. The operating system, installed programs and user files all reside on secondary storage until needed, when they are loaded into RAM.
Types of Secondary Storage
Magnetic storage uses a read/write head to magnetise tiny areas of a spinning magnetic disc (platter). Each area represents a binary 1 or 0 depending on its magnetic orientation.
- Examples: HDD (hard disk drive), magnetic tape
- Capacity: very high (several TB common in HDDs)
- Speed: slower than SSDs — physical movement of read/write head introduces latency
- Durability: vulnerable to physical shock (moving parts); not suitable for portable devices
- Cost: lower cost per gigabyte than SSDs
- Use cases: desktop PCs, server backup drives, archival storage
Optical storage uses a laser to read and write data. A laser burns pits (depressions) and lands (flat areas) into the disc surface; a lower-power laser reads them back. Pits and lands represent 0s and 1s.
- Examples: CD, DVD, Blu-ray
- Capacity: low–medium (CD ~700 MB; DVD ~4.7 GB; Blu-ray ~25 GB)
- Speed: slow compared to HDD and SSD
- Durability: resistant to magnetic fields; discs can be scratched; no moving parts in the disc itself
- Cost: cheap per disc; reader/writer needed
- Use cases: distributing software and films, long-term archival (write-once CDs/DVDs)
Solid state storage uses flash memory chips — no moving parts. Data is stored as electrical charges in transistor cells. Because there are no mechanical components, access times are much faster than HDDs.
- Examples: SSD, USB flash drive, SD card, smartphone storage
- Capacity: high (consumer SSDs typically 256 GB–4 TB)
- Speed: significantly faster than HDD — no seek time
- Durability: very robust — resistant to shock, vibration, magnetic fields; ideal for portable devices
- Cost: higher cost per gigabyte than HDD, but falling rapidly
- Use cases: laptops, smartphones, tablets, boot drives where fast access is critical
Comparison Summary
| Property | Magnetic (HDD) | Optical (DVD) | Solid State (SSD) |
|---|---|---|---|
| Moving parts | Yes | Disc spins when reading | No |
| Speed | Medium | Slow | Fast |
| Capacity | Very high | Low–medium | High |
| Durability (shock) | Poor | Medium | Excellent |
| Cost per GB | Low | Very low | Medium |
| Non-volatile | Yes | Yes | Yes |
Key Takeaways
- Secondary storage is non-volatile — data persists without power; RAM does not.
- Magnetic: spinning platters, cheap per GB, large capacity, vulnerable to shock.
- Optical: laser + pits/lands, low capacity, portable, resistant to magnetic fields.
- Solid state: flash memory, no moving parts, fastest, most durable, highest cost per GB.