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?
One is faulty
One is ECC and one is non-ECC
The slot is dual-channel only
They are different DDR generations with notches in different positions
Which M.2 keying indicates a slot wired for an NVMe (PCIe x4) drive?
M key
B key
A key
E key
A client has four 4 TB drives and wants the most usable storage while still surviving one drive failure. Which RAID level fits best?
RAID 0
RAID 1
RAID 5
RAID 10
What maintenance action should a technician NEVER perform on a solid state drive?
Issue a TRIM command
Update the drive firmware
Defragment the drive
Check the SMART status
Sections you finish are checked off in the contents.