RAM Types & Storage Devices
Key Takeaways
- DDR4 and DDR5 both use 288-pin DIMMs but have notches in different positions, making them physically incompatible — you cannot mix generations in one board.
- DDR5 starts at 4800 MT/s, runs at 1.1V, and adds on-die ECC, versus DDR4 at 2133 MT/s base and 1.2V.
- SSDs have no moving parts and are faster, quieter, and shock-resistant; NVMe (PCIe) drives are far faster than SATA SSDs, which cap near 600 MB/s.
- RAID 0 = striping (no redundancy), RAID 1 = mirroring (50% usable), RAID 5 = striping with one parity drive (min 3), RAID 10 = mirror + stripe (min 4).
- ECC RAM detects and corrects single-bit errors automatically and is used in servers/workstations — not standard consumer desktops.
RAM (Random Access Memory)
DDR (Double Data Rate) Generations
| Specification | DDR3 | DDR4 | DDR5 |
|---|---|---|---|
| Pin Count (DIMM) | 240 | 288 | 288 |
| Pin Count (SODIMM) | 204 | 260 | 262 |
| Voltage | 1.5V | 1.2V | 1.1V |
| Base Speed | 800 MT/s | 2133 MT/s | 4800 MT/s |
| Max Speed | 2133 MT/s | 3200+ MT/s | 8400+ MT/s |
| Per-Module Capacity | up to 16 GB | up to 64 GB | up to 128 GB+ |
| On-Die ECC | No | No | Yes |
Critical Exam Point: DDR generations are NOT interchangeable. DDR4 and DDR5 are both 288-pin, but the notch (key) is in a different position, so DDR5 will not seat in a DDR4 slot. The exam loves the trap that identical pin count implies compatibility — it does not. On-die ECC in DDR5 is internal error correction and is NOT the same as full side-band ECC found on server modules.
RAM Types
| Type | Description | Use Case |
|---|---|---|
| DIMM | Full-size desktop module | Desktops, servers |
| SODIMM | Small-outline laptop module | Laptops, mini PCs, NUCs |
| ECC | Error-Correcting Code | Servers, workstations |
| Registered (RDIMM) | Buffered for stability | Servers with many modules |
| Unbuffered (UDIMM) | No buffer | Consumer desktops |
RAM Channels & Installation
Populating matched pairs in the correctly color-coded slots enables dual-channel (or quad-channel on HEDT boards), which roughly doubles memory bandwidth. A single stick or a mismatched pair forces single-channel mode and lower performance.
- Match specs — type, speed, and voltage to the motherboard QVL
- Install in matched pairs in the labeled A2/B2 slots (consult the manual)
- Avoid mixing speeds/sizes — the system runs at the slowest module's timings
- Handle by the edges, use ESD protection, never touch gold contacts
- Enable XMP/EXPO in UEFI so RAM runs at its rated speed, not the JEDEC base
Storage Devices
HDD (Hard Disk Drive)
| Spec | 3.5-inch (Desktop) | 2.5-inch (Laptop) |
|---|---|---|
| Technology | Spinning magnetic platters | Spinning magnetic platters |
| RPM | 5400 / 7200 / 10000 / 15000 | 5400 / 7200 |
| Interface | SATA III (6 Gbps) | SATA III (6 Gbps) |
| Capacity | up to 20+ TB | up to 5 TB |
| Sequential Read | 100–200 MB/s | 80–160 MB/s |
Higher RPM means faster access but more heat and noise; 15000 RPM drives are enterprise-only. HDDs win on cost-per-gigabyte for bulk storage and backups.
SSD (Solid State Drive)
| Form Factor | Interface | Max Speed | Keying / Connector |
|---|---|---|---|
| 2.5-inch SATA | SATA III | ~550 MB/s | SATA data + power |
| M.2 SATA | SATA III | ~550 MB/s | B+M key |
| M.2 NVMe (Gen 3) | PCIe 3.0 x4 | ~3,500 MB/s | M key |
| M.2 NVMe (Gen 4) | PCIe 4.0 x4 | ~7,000 MB/s | M key |
| M.2 NVMe (Gen 5) | PCIe 5.0 x4 | ~12,000 MB/s | M key |
| mSATA | SATA III | ~550 MB/s | mSATA edge |
Exam Tip: M.2 keying decides what a slot accepts. B key = SATA, M key = NVMe (PCIe x4), B+M key = the drive fits both kinds of slot. A SATA M.2 drive will physically seat in an M-key NVMe slot only if the slot is wired for SATA — always check the board manual. M.2 length codes like 2280 mean 22 mm wide x 80 mm long.
SSD vs. HDD
| Feature | SSD | HDD |
|---|---|---|
| Speed | Up to ~12,000 MB/s (NVMe Gen 5) | 100–200 MB/s |
| Moving parts | None — shock resistant | Platters/heads — fragile |
| Noise | Silent | Audible spin/click |
| Cost/GB | Higher | Lower |
| Wear metric | TBW (terabytes written) | Mechanical MTBF |
Never defragment an SSD — it adds wear with no benefit. Use TRIM instead, which the OS issues automatically to maintain write performance.
RAID (Redundant Array of Independent Disks)
| Level | Description | Min. Drives | Usable Capacity | Fault Tolerance |
|---|---|---|---|---|
| RAID 0 | Striping | 2 | 100% | None — any failure loses all data |
| RAID 1 | Mirroring | 2 | 50% | 1 drive can fail |
| RAID 5 | Stripe + distributed parity | 3 | (n−1) drives | 1 drive can fail |
| RAID 6 | Stripe + double parity | 4 | (n−2) drives | 2 drives can fail |
| RAID 10 | Mirror + stripe (1+0) | 4 | 50% | 1 per mirror set |
RAID exam shortcuts
- RAID 0 — max speed/capacity, zero protection. Never for critical data.
- RAID 1 — simple mirror, survives one failure, 50% overhead.
- RAID 5 — most popular server choice; good capacity efficiency with single-drive protection. Write penalty from parity calculation.
- RAID 10 — best blend of speed and redundancy, but you only get half your raw capacity and need at least four drives.
Scenario: A small business wants maximum usable space with single-disk fault tolerance across four 4 TB drives. RAID 5 yields ~12 TB usable (n−1); RAID 10 would yield only ~8 TB.
Two RAM modules are both 288-pin DDR modules, yet one will not seat in the slot that holds the other. What is the most likely reason?
Which M.2 keying indicates a slot wired for an NVMe (PCIe x4) drive?
A client has four 4 TB drives and wants the most usable storage while still surviving one drive failure. Which RAID level fits best?
What maintenance action should a technician NEVER perform on a solid state drive?