Learning Objectives
By the end of this topic you will be able to:
Explain how HDDs store data using magnetic technology
Explain how SSDs store data using NAND flash memory
Describe optical storage: CD, DVD, Blu-ray
Compare storage technologies on speed, capacity, cost and use case
HDD
Hard Disk Drives (HDD)
HDDs store data by magnetising regions on spinning magnetic platters. Electromagnetic read/write heads float nanometres above the platter surface and detect or flip magnetic polarities.
Platters: multiple rigid circular disks coated in magnetic material, spinning at 5,400–15,000 RPM
Tracks and sectors: platters divided into concentric tracks, each split into sectors (usually 512 B or 4 KB)
Actuator arm: moves heads radially to access any track; seek time adds latency
Capacity: up to 20+ TB. Cost: cheap per GB. Speed: 100–200 MB/s. Fragile — moving parts susceptible to shock.
SSD
Solid State Drives (SSD)
SSDs store data in NAND flash memory cells — charge is trapped in floating-gate transistors. SLC (1 bit/cell), MLC (2 bits/cell), TLC (3 bits/cell), QLC (4 bits/cell).
Advantages over HDD
No moving parts → shock-resistant, silent, lower power. Much faster: 500 MB/s (SATA) to 7,000 MB/s (NVMe PCIe). Near-instant random access (no seek time).
Disadvantages
More expensive per GB. Limited write endurance (P/E cycles) — cells wear out after 1,000–10,000 writes. Data loss on long-term power loss.
Optical
Optical Storage
Optical discs store data as a pattern of pits and lands on a reflective surface. A laser reads the disc: no reflection (pit transition) = 1; full reflection (land) = 0.
CD
700 MB. 780 nm laser. Single layer. Used for audio, small software.
DVD
4.7–17 GB. 650 nm laser. Single/dual layer, single/dual side. Video distribution.
Blu-ray
25–100 GB. 405 nm blue-violet laser. Shorter wavelength allows smaller pits → higher density.
Write-once (CD-R, DVD-R): dye layer permanently altered. Rewritable (CD-RW, DVD-RW): phase-change material. Sequential access is a limitation; fast sequential reads only.
Magnetic Tape
Magnetic Tape
Magnetic tape stores data sequentially on long strips of magnetically-coated tape. Data is read/written by heads as the tape passes at high speed.
Access type: sequential only — to access a record, the tape must be wound to that position (high latency).
Capacity: up to 45 TB per cartridge (LTO-9). Extremely cheap per GB.
Use case: archival/backup storage where access time is unimportant. Used by cloud providers and broadcasters for long-term data retention.
Common Mistakes
Don’t Lose Marks
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Saying Blu-ray uses a blue laser just because it’s blue — the key point is that a shorter wavelength (405 nm) allows smaller pits, increasing storage density.
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Describing SSDs as having no memory — SSDs use NAND flash memory. They have memory; they just don’t have spinning disks or moving read heads.
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Forgetting that magnetic tape is sequential access — you cannot jump directly to any record. This is its key disadvantage and must be mentioned when comparing to disk-based storage.