HDD — Hard Disk Drive
HDDs store data on magnetic platters that spin at high speed (typically 5400 RPM or 7200 RPM for consumer drives; 10,000–15,000 RPM for enterprise). A read/write head moves across the spinning platters to access data. Because data access requires physical movement, HDDs are significantly slower than SSDs, especially for random read/write operations.
HDDs connect via SATA in modern systems. Despite being slower, HDDs remain cost-effective for large-capacity storage — you can get 8 TB HDDs for a fraction of the cost of an equivalent SSD. They're commonly used for backup drives, NAS systems, and secondary storage where speed isn't critical.
Key failure modes: Head crashes (head touches platter), bearing failure, motor failure. Clicking or grinding sounds from a hard drive are warning signs of imminent failure. HDDs are sensitive to physical shocks while operating.
SSD — Solid State Drive
SATA SSDs use NAND flash memory — no moving parts. Because there's no physical movement, access times are dramatically lower than HDDs. A SATA SSD can read sequential data at around 500–560 MB/s, compared to 80–160 MB/s for an HDD. Random read/write performance is even more dramatically improved — this is why systems feel so much faster after an SSD upgrade.
SATA SSDs use the same SATA III interface as HDDs and come in the same 2.5-inch form factor, making them direct HDD replacements in laptops and desktops. The SATA interface is the bottleneck — SATA III's 600 MB/s maximum is nearly saturated by modern SSDs, which is why NVMe was developed.
Flash cell types: SLC (1 bit/cell — fastest, most durable, most expensive), MLC (2 bits), TLC (3 bits — most common consumer SSD), QLC (4 bits — cheapest, slowest, least durable).
NVMe (Non-Volatile Memory Express) is a protocol designed specifically for flash storage over the PCIe bus — bypassing the SATA controller entirely. PCIe has far higher bandwidth than SATA, allowing NVMe SSDs to achieve sequential read speeds of 3,500–7,000+ MB/s (Gen 4/Gen 5). This is 6–14× faster than SATA SSD.
NVMe SSDs typically come in the M.2 form factor and plug directly into an M.2 slot on the motherboard. They also come in PCIe add-in card (AIC) format for desktops. The NVMe protocol is also much more efficient than AHCI (the protocol SATA SSDs use) — NVMe supports up to 65,535 command queues vs AHCI's single queue of 32 commands.
Thermal note: NVMe SSDs run hot under sustained load — thermal throttling is common in laptops and compact builds without adequate cooling. Many M.2 slots in desktops come with heatsinks for this reason.
Form Factors and Interfaces
| Form Factor | Size | Compatible With | Exam Note |
|---|---|---|---|
| 3.5-inch | Standard desktop | HDDs only — desktops and NAS enclosures | Requires 4-pin Molex or SATA power; fits desktop drive bays |
| 2.5-inch | Laptop size | HDDs and SATA SSDs — laptops and desktops with adapter bracket | Thinner than 3.5" — most laptop drives are 2.5" |
| M.2 | Small stick (22mm wide) | SATA SSDs OR NVMe SSDs — determined by the drive's key type | M.2 is a form factor, not a protocol — check if slot is SATA, NVMe, or both |
| mSATA | Mini card | Older SATA SSDs — mostly legacy laptops and embedded systems | Replaced by M.2 — still appears on A+ exam as legacy connector |
M.2 Key Types
M.2 drives have a notch called a "key" that determines compatibility. This is a common A+ exam topic because M.2 slots and drives must have matching keys:
| Key | Protocol Support | Exam Note |
|---|---|---|
| M-key | NVMe (PCIe) and SATA | Most common on modern motherboards — accepts NVMe drives and some SATA M.2 drives |
| B-key | SATA only (and older PCIe ×2) | Older slot type — won't accept modern NVMe drives that require PCIe ×4 |
| B+M key | SATA (fits both B and M slots) | Notched on both ends — SATA M.2 drives typically have B+M keying for maximum compatibility |
M.2 is a form factor. NVMe is a protocol. A drive can be M.2 and use the SATA protocol (slower, ~500 MB/s). Another drive can be M.2 and use the NVMe protocol (faster, 3,500+ MB/s). The exam will give you a scenario where a user installs an M.2 drive into a new M.2 slot and it doesn't work — the most likely cause is a protocol mismatch: a SATA M.2 drive in an NVMe-only slot, or vice versa.
Also: NVMe ≠ PCIe AIC — NVMe can come in M.2 or PCIe add-in card form. The protocol (NVMe) and the form factor (M.2 or AIC) are separate characteristics.
RAID — Redundant Array of Independent Disks
RAID combines multiple drives to provide either fault tolerance (data survives a drive failure) or improved performance (or both). The A+ exam tests RAID 0, 1, 5, and 10 — you must know how each works, the minimum drives required, and what happens when a drive fails.
Failure behaviour: If any single drive fails, all data is lost — there is no redundancy. RAID 0 trades safety for speed entirely.
Use case: Video editing scratch disks, gaming drives, any workload prioritising speed over data safety. Not appropriate for any data you cannot afford to lose.
Failure behaviour: If one drive fails, the other contains a complete copy of all data — the system continues operating without data loss. Replace the failed drive and the array rebuilds automatically.
Use case: OS drives, critical data that must survive a single drive failure. Common in servers and workstations where downtime is unacceptable.
Failure behaviour: The array can survive the failure of one drive. The system continues operating (in a degraded state) until the failed drive is replaced and the array rebuilds. If a second drive fails during rebuild — all data is lost.
Use case: NAS drives, file servers, small business storage — good balance of capacity, performance, and redundancy.
Failure behaviour: Can tolerate the failure of one drive per mirror pair. With 4 drives (2 pairs), up to 2 drives can fail — as long as they're not both from the same mirror pair. If both drives in a mirror pair fail, data is lost.
Use case: Database servers, high-transaction workloads requiring both speed and redundancy. The premium choice when performance and fault tolerance are both required.
RAID is NOT a backup. This is the most important RAID concept on the exam. RAID protects against drive failure — it does not protect against accidental deletion, ransomware, or corruption (which replicates to all mirrors instantly). Backups and RAID serve different purposes.
RAID 0 has no fault tolerance — any drive failure destroys all data. RAID 1 and RAID 5 can each survive one drive failure. RAID 10 can survive one failure per mirror pair. The exam frequently asks "which RAID level provides redundancy with the best write performance?" → RAID 10.
Storage Speed Comparison
| Storage Type | Interface | Typical Read Speed | Relative Speed |
|---|---|---|---|
| HDD (5400 RPM) | SATA III | 80–120 MB/s | Slowest |
| HDD (7200 RPM) | SATA III | 120–160 MB/s | Slow |
| SATA SSD | SATA III | 500–560 MB/s | Fast |
| NVMe SSD (Gen 3) | PCIe 3.0 ×4 | 3,000–3,500 MB/s | Very fast |
| NVMe SSD (Gen 4) | PCIe 4.0 ×4 | 5,000–7,000 MB/s | Extremely fast |
Exam Scenarios
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