The physical devices computers use to permanently hold data — a quick-reference cheat sheet, not a deep dive.
| Category | Notable Facts |
|---|---|
| Non-volatile | Retains data without power |
| Magnetic | HDD, tape |
| Optical | CD, DVD, Blu-ray |
| Solid-state | SSD, flash/USB drives |
| Feature | Notable Facts |
|---|---|
| RPM (Revolutions Per Minute) | Platters spin at 4,500–15,000 RPM |
| R/W (Read/Write) head | Floats 3–6nm above the platter — thinner than a strand of DNA |
| Head crash | Catastrophic head-to-platter contact |
| PMR (Perpendicular Magnetic Recording) / SMR (Shingled Magnetic Recording) | Modern recording techniques for higher density |
| HAMR (Heat Assisted Magnetic Recording) / MAMR (Microwave Assisted Magnetic Recording) | Future roadmaps toward 50–60TB drives |
| Interface | SATA (Serial Advanced Technology Attachment, consumer) vs SAS (Serial Attached SCSI, enterprise) |
| Size | Notable Facts |
|---|---|
| 3.5" | Desktop standard — higher capacity, used in desktops & servers |
| 2.5" | Laptop standard — smaller, lower power draw, used in laptops & external drives |
| Type | Notable Facts |
|---|---|
| SSD | No moving parts — silent, shock-resistant, fast access |
| Hybrid | Flash cache + spinning platters for bulk storage |
| Type | Interface | Notable Facts |
|---|---|---|
| SATA (Serial Advanced Technology Attachment) SSD | SATA | 2.5" form factor, same connector as an HDD; capped by SATA III speed (~550 MB/s) |
| M.2 SSD | SATA or NVMe | Small stick that plugs directly into a motherboard M.2 slot; M.2 is a physical form factor, not a protocol — speed depends on whether it runs SATA or NVMe underneath |
| NVMe (Non-Volatile Memory Express) SSD | PCIe | Runs the NVMe protocol over PCIe (PCI Express) lanes (usually via M.2); far faster than SATA — several GB/s vs ~550 MB/s |
| Concept | Notable Facts |
|---|---|
| Laser wavelength | Shorter wavelength = more data (Blu-ray gets its name from its blue-violet laser) |
| Pits & lands | Data stored as micro pits/lands, read via reflected light |
| Format | Laser | Capacity (single layer) |
|---|---|---|
| CD (Compact Disc) | 780nm (red) | 0.7 GB |
| DVD (Digital Versatile Disc) | 650nm (red) | 4.7 GB |
| Blu-ray | 405nm (blue-violet) | 25 GB (50 GB dual-layer) |
| Feature | Notable Facts |
|---|---|
| Access | Sequential only — slow to locate data |
| Use case | Enterprise backup — durable, long shelf life |
| Feature | Notable Facts |
|---|---|
| Cell technology | Floating-gate transistor — same underlying tech as ROM |
| Write cycles | Roughly 10,000 to 1,000,000 before wear-out |
| Floppy disk history | 8in → 5.25in → 3.5in |
| SD (Secure Digital) card capacity | Up to 2TB today |
| Metric | Notable Facts |
|---|---|
| Seek time | Time to move the head to a location — lower is better |
| Access time | Time from request to data delivery — lower is better |
| RPM (Revolutions Per Minute) | Higher spin speed = faster drive |
| Capacity | Raw storage space, measured in bytes |
| Interface | SATA (Serial Advanced Technology Attachment, consumer) vs SAS (Serial Attached SCSI, enterprise) |