1.4 · Storage Devices
Goal: compare the major secondary storage devices on access mode, volatility, transfer rate, capacity, and pick the right one for a scenario.
What "secondary storage" means
Anything that persists when the power goes off and is not the running RAM. Used for the OS, applications, documents, photos and backups.
The five categories you must know
| Device | Access | Volatile? | Typical transfer rate | Typical capacity | Notes |
|---|---|---|---|---|---|
| Magnetic Hard Disk (HDD) | Random | No | 100–200 MB/s | up to ~20 TB | Cheap per GB; moving parts → fragile |
| Optical Disk (CD, DVD, Blu-ray) | Random | No | ~50 MB/s | up to 100 GB (BD) | Distribution media; archival |
| Flash memory (USB, SSD, SD card) | Random | No | SATA SSD 500 MB/s, NVMe SSD up to 7 GB/s | up to ~8 TB | No moving parts; quiet, durable |
| Magnetic Tape | Sequential | No | 200–400 MB/s | up to ~20 TB+ per cartridge | Backup; cheap per GB; slow seek |
| Network / Cloud storage | Random over network | No | Depends on network | Virtually unlimited | OneDrive, iCloud, Dropbox |
Comparison axes you must master
Access mode
- Random / direct → HDD, SSD, optical, USB, NAS — jump to any record.
- Sequential → tape — read in order from the start.
Volatility
- All listed devices are non-volatile. (Volatile = RAM, cache, registers.)
Capacity vs cost
- Per-GB cost order (cheap → expensive):
- Tape < HDD < SSD < High-end NVMe SSD < RAM
- Trend: every five years SSDs drop in price; HDDs hold their cost edge for cold storage.
Speed
- NVMe SSD > SATA SSD > HDD > Optical > Tape.
Mobility
- USB > SD card > External SSD > Optical disk > External HDD > Tape > Cloud (instant from anywhere with Internet).
Trends to mention in essays
- Sizes shrink, capacities grow. A modern 1 TB microSD card is smaller than a thumbnail.
- SSDs increasingly replace HDDs in laptops and even data centres for hot data.
- Cloud storage has democratised off-site backup.
- Tape is far from dead — enterprises and hyperscale clouds still use LTO tapes for cold storage because of unmatched per-GB cost.
Worked example · Designing storage for a school
| Need | Best choice | Reason |
|---|---|---|
| OS + programs on classroom PCs | SATA / NVMe SSD | Fast boot, snappy apps |
| Shared student files | Network attached storage (NAS) with HDDs | Cheap per GB, accessible from any PC |
| Daily backup off-site | Cloud (Microsoft 365 / Google Workspace) | Versioning, disaster recovery |
| Long-term archive of CCTV footage | External HDDs or tape | Cheapest per GB |
| Sharing a class video at school assembly | USB flash drive | Portable, plug-and-play |
Common student mistakes
- Calling SSDs "Solid State HDD" — they are not HDDs, no spinning platters.
- Treating cloud storage as a type of device — it is a service running on someone else's storage.
- Forgetting that optical disks (CD/DVD) are still required for some legacy systems.
- Calling tape "obsolete" — it is alive and well in enterprise backup.
Exam-style question
Q (4 marks): Compare an SSD with a magnetic HDD in terms of speed, durability and cost per GB. Suggest one situation where an HDD remains preferable.
Sample answer:
- Speed: SSDs use flash memory with no moving parts, achieving transfer rates of 500 MB/s (SATA) to 7 GB/s (NVMe). HDDs rely on rotating platters and reach ~100–200 MB/s, with seek delays.
- Durability: SSDs have no moving parts, so they tolerate vibration and drops; HDDs can fail if dropped while spinning.
- Cost per GB: HDDs remain considerably cheaper per GB than SSDs.
When HDDs win: archival storage of large quantities of cold data (e.g. CCTV footage, backup vaults) where capacity per dollar matters more than speed.
Key takeaways
- Five families: HDD, optical, flash (SSD/USB), tape, cloud.
- Compare on access mode, volatility, transfer rate, capacity, cost, mobility.
- Choose based on workload, not fashion.
➡️ Next: 1.5 Input/Output Devices