An HDD is an auxiliary storage device that spins a magnetic disk and reads and writes data with a magnetic head that moves across it.
Move the head to the target position
Wait for the platter to rotate the target spot into place
Read or write data magnetically
Once they line up, the head reads or rewrites the direction of magnetization.
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It is a non-volatile device that keeps its contents when the power is off, and it serves mainly as a PC's auxiliary storage. Every read or write has to wait for the head to move and the platter to turn, which adds mechanical delay. It is cheap per unit of capacity and easy to make large, but its moving parts leave it vulnerable to shock and vibration.
AnalogyThink of a record player. Just as you lower the needle onto a spinning record to find a track, an HDD moves its head to find where the data sits. And like a needle, the head does not take kindly to being jolted.
How to rememberHard Disk means a rigid disk, a name chosen to contrast it with the floppy disk, which used a flexible one.
Exam points
ExamAn HDD can move its head straight to any spot on the disk, so it reads and writes without stepping through the data from the start. This is random access (ランダムアクセス).
ExamBecause an HDD waits for the head to move and the platter to turn, its access time is longer than that of semiconductor storage such as main memory (主記憶) or cache memory (キャッシュメモリ). The lower a device sits in the memory hierarchy (記憶階層), the slower it is.
An SSD is an auxiliary storage device that records data in flash memory and has no parts that spin or move.
It is a non-volatile device that keeps its contents when the power is off, and in the memory hierarchy it counts as auxiliary storage. It reads and writes by addressing the data's location electrically, so there is no waiting for a head to move or a platter to turn as in an HDD.
AnalogyPicture the same kind of chips found in a USB flash drive, fixed inside the PC and scaled up to a much larger capacity.
How to rememberSolid State refers to a solid physical state, signaling that the device has no moving mechanical parts and is made only of solid electronic components.
Exam points
ExamAn SSD uses little power because it needs neither a motor to spin a disk nor a mechanism to carry a head.
ExamThe flash memory (フラッシュメモリ) inside an SSD can be rewritten only a limited number of times, and that limit affects the device's lifespan.
ExamSecure Erase is a method that commands an SSD's built-in erase function to clear its storage area in one go. It also clears areas where data can remain after files are deleted or overwritten.
A USB flash drive (USBメモリ) is a portable storage medium with built-in flash memory that plugs directly into a USB port.
Small, light, and ready the moment you plug it in, it is used to move data between devices and to store files temporarily. Its contents stay intact even after you unplug it and the power is cut.
How to rememberUSB stands for Universal Serial Bus, originally the name of a general-purpose standard for connecting devices. The drive takes its name from the port it plugs into.
Exam points
ExamThe component that holds data inside a USB flash drive (USBメモリ) is semiconductor flash memory (フラッシュメモリ).
An SD card (SDカード) is a storage medium that packs flash memory into a thin, small card and goes into a device's card slot.
Digital cameras, smartphones, and similar devices use it widely to record photos and video. It comes in different sizes, such as the smaller microSD, and in capacity-based standards such as SDHC and SDXC.
AnalogyIt took over the role that film played in film cameras: it records what you shoot, and you can take it out and move it to another device.
How to rememberSD stands for Secure Digital. The word Secure comes from its built-in copyright protection features.
USB Flash Drive vs. SD Card
Aspect
USB Flash Drive
SD Card
How it records
Flash memory
Flash memory
Key differenceWhere it plugs in
USB port
Card slot
Main use
Moving data between PCs
Recording on cameras and smartphones
Key points
Of the four in this unit, only the HDD records magnetically; the rest use flash memory.
In the memory hierarchy, devices higher up are faster to access and cost more per unit of capacity, while those lower down are slower, cheaper, and larger.
Auxiliary storage keeps its contents when the power is off and serves to hold programs and data for the long term.
HDDs and SSDs are mainly devices built into equipment, while USB flash drives and SD cards are media you unplug and carry around.