2026-09-24 · class-notes, a-plus, comptia, messer, hardware, storage, ssd, nvme, hard-drive, flash
Full class notes for Professor Messer A+ 220-1201 objective 3.4: hard drives and RPM, SSDs, PCIe and NVMe, SAS, mSATA and M.2 keying, flash memory formats, and optical drives.
Class notes · Professor Messer, CompTIA A+ 220-1201 Core 1 · Section 3, objective 3.4 (Storage Devices)
Quick reference: the short version of this lesson is the Storage Devices cheat sheet in A+ Core 1 section 3. It opens objective 3.4, following the memory lessons (objective 3.3) in this section — where memory was volatile working storage, this lesson covers the non-volatile devices that keep data after power is removed.
By the end of these notes you should be able to:
Memory (RAM) is volatile — when a system powers off, everything held in memory disappears. To keep data available afterwards, a system needs storage that survives without power. Many technologies fill this role: hard drives, solid-state drives, flash drives, memory cards, optical drives, and others covered in this lesson.
A hard drive is a magnetic storage device with rapidly spinning platters inside, where data is physically stored. It uses random access: data can be stored anywhere on the drive and retrieved immediately by addressing the location directly, rather than reading through everything before it.
Opening a hard drive (something you should never do outside a dust-free environment) reveals several moving parts:
| Component | Role |
|---|---|
| Platter | The spinning disk where data is magnetically stored |
| Spindle | The motor assembly that spins the platter(s) |
| Actuator | Moves the arm across the platter |
| Arm | Carries the read/write head |
| Read/write head | Reads and writes data on the platter surface just benea |
Because these are mechanical, moving components, any one of them can fail at any time — a hard drive's mechanical nature is its central reliability weakness compared with solid-state alternatives.
+------------------------------------------------------------------------------+
| |
| Hard drive, simplified cutaway view |
| |
| ____________________________ |
| / \ |
| | platter (spinning) | <-- spindle motor underneath |
| | . . . . | |
| | | |
| |___________ ________ ________| |
| \/ |
| [ actuator arm ] |
| | |
| [ read/write head ] <-- rides just above the surface |
| |
+------------------------------------------------------------------------------+
Drives spin at different rates, most commonly 5,400, 7,200, 10,000, or 15,000 RPM (revolutions per minute). The read/write head stays in one position while the platter spins beneath it, so the drive must wait for the platter to rotate the wanted data underneath the head. Faster rotation means lower latency — a 15,000 RPM drive reaches the data it needs sooner than a 5,400 RPM drive, because the wait for the right part of the platter to come around is shorter.
Many drives contain more than one platter, stacked together, and there is a read/write head and actuator arm on both the top and bottom surface of each platter — multiplying the storage capacity and the read/write hardware working in parallel within one drive.
Desktop systems have plenty of internal room, so drive size matters less there. Mobile devices benefit directly from smaller, lighter drives:
| Form factor | Width | Typical use |
|---|---|---|
| 3.5 inch | 3.5 in | Desktop computers |
| 2.5 inch | 2.5 in | Laptops, mobile devices |
| SSD (non-2.5"/3.5" form) | 22 mm | Modern compact systems |
Note (beyond this lesson): 3.5-inch and 2.5-inch both refer to standardised width dimensions defined for the drive bay the device sits in, not to storage capacity — a 2.5-inch drive is not automatically lower capacity than a 3.5-inch one, just physically smaller.
SSDs — solid-state drives — use non-volatile memory rather than spinning platters, so they have no moving parts. Open one up and you will mostly see memory modules, not mechanical components.
SSDs are many times faster at reading and writing than a traditional hard drive. Replacing a hard drive with an SSD, with no other changes, typically produces a substantial improvement in overall system throughput — this is one of the most effective single upgrades available for an ageing system.
Moving from hard drives to SSDs was itself a large performance gain, but it soon exposed a new limit: the traditional SATA connection's maximum throughput was itself becoming the bottleneck. SSDs were capable of more than SATA could deliver.
One solution was to connect the SSD directly to the PCI Express bus rather than through SATA — commonly via an adapter card that plugs into a PCIe slot, with the SSD installed on the card itself. The motherboard supplies both power and the much higher throughput of the PCIe bus.
SATA tops out at 6 Gbps (SATA revision 3, as covered in the network cables lesson). PCI Express offers considerably more.
Caution: the transcript states PCI Express provides "around 64 gigabits per second per lane." This overstates a single lane's throughput. A single PCIe 4.0 lane runs at roughly 16 Gbps, and a single PCIe 3.0 lane at roughly 8 Gbps — nowhere near 64 Gbps per lane. The 64 Gbps figure is close to the aggregate throughput of a PCIe 4.0 x4 link (four lanes together, the configuration most NVMe SSDs actually use) rather than one lane on its own. The comparison against SATA's 6 Gbps still stands — PCIe storage is genuinely much faster — but the "per lane" attribution needs correcting to "aggregate, over the typical four-lane link."
Traditional SATA storage uses a standard called AHCI — Advanced Host Controller Interface — which governs how data moves between the drive and memory. AHCI was designed around the assumptions and limitations of spinning-disk, SATA-connected storage.
To take proper advantage of the PCI Express bus, a new communication method was created: NVMe — Non-Volatile Memory Express. NVMe offers very low latency and higher throughput, because it connects directly to the PCIe bus rather than routing through the older AHCI/SATA model — and it works this way even inside a laptop, where a full-size PCIe adapter card would not fit.
The most common way to reach the PCIe bus in laptops and modern desktops is the M.2 interface (covered in more depth in section 6). An NVMe drive on M.2 offers a theoretical transfer speed in the region of 20 Gbps — well beyond SATA's 6 Gbps ceiling.
Some systems still use traditional spinning hard drives and need better throughput than SATA provides for them. Serial Attached SCSI (SAS) is a serialised version of the long-established SCSI protocol, letting systems retain SCSI's protocol advantages for controlling and managing drive data while gaining the benefits of a modern serial connection.
SAS reaches approximately 22.5 Gbps — substantially faster than SATA's 6 Gbps — and future SAS revisions are expected to increase this further. I checked this figure: it matches the published line rate for 24G SAS (SAS-4), which is documented at 22.5 Gb/s per lane, so the transcript's number is accurate.
A SAS drive's interface looks a great deal like a SATA drive's: two separate connectors, data on one side, power on the other, in essentially the same physical arrangement. Placed side by side, a SATA drive and a SAS drive look very alike in form factor.
The connectors themselves are slightly different, though — a deliberate design choice to prevent accidentally plugging a SATA drive into a SAS configuration, or vice versa.
Exam tip: SAS drives are typically found in large storage arrays of spinning hard drives, where the SCSI protocol's array-management strengths and the higher throughput over SATA both matter at scale.
As storage devices got smaller and the interfaces connecting them needed to get faster, two additional interface technologies emerged alongside standard SATA: mSATA and M.2.
mSATA stands for mini SATA. It took the same underlying SATA connections and repackaged them into a smaller physical form factor. mSATA served as a useful stopgap between full-size SATA connectivity and the eventual move to M.2 — it addressed board-space constraints and offered a path to a somewhat higher-speed bus, without yet being the final answer.
M.2 has become the dominant interface for modern storage devices, particularly SSDs. It needs no separate data or power cables — the M.2 drive plugs directly into the M.2 slot on the system board. Because M.2 connects directly to the system bus, it can take full advantage of PCI Express speeds.
Not every M.2 interface supports the same type of connectivity, and the way this is indicated is through small keys — notches at the connection end of the drive. The two commonly referenced keys are:
| Key | Note |
|---|---|
| B key | One keying position |
| M key | Another keying position |
| B+M key | Some drives support both, fitting either type of slot |
Checking motherboard documentation for which key (or keys) its M.2 slots support is necessary before assuming any given M.2 drive will work in it — the interface may otherwise support only SATA-style throughput, or genuinely fast NVMe throughput, depending on how the slot and drive are keyed and wired.
+------------------------------------------------------------------------------+
| |
| M.2 drive, connector end (simplified) |
| |
| [==============contacts==============] |
| ^ ^ |
| B key M key (positions of the notch/notches) |
| |
| B-keyed only: notch at the B position |
| M-keyed only: notch at the M position |
| B+M keyed: notches at BOTH positions -- fits either type of slot |
| |
+------------------------------------------------------------------------------+
Installation is simple and fast: slide the drive into the M.2 slot at an angle, then fasten it down to the system board with a small retaining screw. No separate cabling is involved at all.
Scenario: you are asked to add or upgrade storage in an older laptop and are not sure which interfaces it actually supports.
Flash drives store information in a very small physical form using EEPROM — Electrically Erasable Programmable Read-Only Memory. This is non-volatile: remove the drive from power entirely, and the stored data remains available.
EEPROM supports only a certain number of write cycles. Beyond that limit, the memory stops accepting new writes — though it may still be readable. This is a genuine, permanent limitation of the technology, not a fault.
Flash drives are not recommended as sole archival or backup media, for two compounding reasons: the limited write endurance described above, and their small physical size, which makes them easy to lose. The practical guidance is straightforward — if information matters, keep a backup in another location as well as (or instead of) a flash drive.
| Format | Note |
|---|---|
| USB flash drive | The most common and widely seen form |
| CF (CompactFlash) | One of the original, comparatively large flash format |
| SD (Secure Digital) | Common in mobile devices |
| miniSD / microSD | Smaller variants for very compact devices |
| xD-Picture Card | Found in some older digital cameras |
Optical drives see little use in production systems today, but a substantial amount of historically stored information exists on optical media, making the ability to read it still relevant.
Optical drives store data as tiny microscopic bumps on the disc, written and later read using a laser beam. This method can store a great deal of information in physically compact media.
Reading and writing via a laser is comparatively slow next to a hard drive or SSD. In exchange, optical media takes up very little room while storing substantial data, which made it a genuinely good choice for archiving data for later retrieval, even if not for active working storage.
Common optical formats include CD-ROM, DVD-ROM, and Blu-ray. Some systems have a built-in optical drive; others require an external reader, which typically connects over USB — a practical option if you need to retrieve information from older optical archives on a modern system without a built-in drive.
Storage device choice affects several defender-relevant concerns beyond raw speed and capacity.
| Term | Meaning |
|---|---|
| Volatile | Loses its contents when power is removed (describes RAM) |
| Non-volatile | Retains its contents without power (describes storage) |
| Platter | The spinning disk inside a hard drive where data is stored |
| Spindle | The motor assembly spinning a hard drive's platters |
| Actuator | The mechanism moving the read/write arm across the platter |
| RPM | Revolutions per minute; a hard drive's rotational speed |
| SSD | Solid-state drive; non-volatile memory storage with no moving parts |
| PCIe | PCI Express; a high-throughput bus SSDs can connect to directly |
| AHCI | Advanced Host Controller Interface; the standard for SATA storage |
| NVMe | Non-Volatile Memory Express; low-latency storage over PCIe |
| SAS | Serial Attached SCSI; a fast, serialised SCSI-based storage interface |
| mSATA | Mini SATA; a smaller SATA form factor, a stopgap before M.2 |
| M.2 | A cable-free storage interface connecting directly to the system bus |
| M.2 keying | Notches (B key, M key, or both) indicating an M.2 slot's support |
| Flash memory | Non-volatile memory used in compact removable storage devices |
| EEPROM | Electrically Erasable Programmable Read-Only Memory; the basis of fl |
| Optical drive | A drive reading/writing data as laser-etched marks on a disc |