Describe internal and external types of secondary memory storage

Internal hard drives (SSD, HDD) and eMMCs

Internal Hard Disk Drives (HDDs)

• Stores data on magnetic disk platters.
• Most common internal storage in desktops, laptops, servers.
• High capacity and reliable, but comparatively slow (avg sustained transfer 100–200 MB/s).
• Called “hard” because it uses rigid, non-flexible magnetic platters (磁盘) — unlike old floppy disks.
  • Platters spin at high speed; a read/write head (读写磁头) moves across their surfaces. Data is stored/retrieved by magnetizing tiny regions on the platter. 读写头通过磁化盘面上的微小区域来记录或读取数据，就像在一张飞速旋转的金属唱片上”刻字”和”读字”。

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Solid-State Drives (SSDs)

• Uses non-volatile flash memory → much faster access, lower power, greater durability than HDDs because it has no moving parts.
• Same purpose as HDDs but faster and lower power. No moving parts → quicker access, lower latency.
• Transfer rates can far exceed HDDs — up to 2,500–7,000 MB/s.

How it works

• Stores data using NAND flash memory (non-volatile). Data kept in floating-gate transistors (memory cells, 浮栅晶体管) in a grid. Each cell stores a bit (or multiple bits) by holding/releasing charge.
• NAND flash structured into blocks containing smaller pages. Data is written per page but erased per block. Reading senses charge in the floating-gate transistor.
• Internal SSD controller handles data transfer, error correction, and wear-leveling (磨损均衡) — algorithm spreading write/erase cycles evenly to extend lifespan (flash cells have limited write cycles).
• NAND types: SLC, MLC, TLC, QLC — storing 1/2/3/4 bits per cell respectively (cost/speed/endurance tradeoffs).

R/W/E Cycle

• Each memory cell = tiny transistor with a floating gate that traps/releases charge.
• Write: SSD controller applies voltage to inject/remove charge in floating gate. Charge state determines binary value (or multiple bits for MLCs).
• Read: senses charge in floating gate → determines 0 or 1.
• Erase: floating gates of all cells in a block are cleared simultaneously. So erase = per block; write = per page.

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Embedded Multimedia Cards (eMMCs)

• NAND flash memory storage integrating memory, controller, and multimedia card interface in a single package on the motherboard.

Characteristics

• Solid-state but generally slower than SSDs (lower sequential R/W speeds and IOPS).
• Typical capacity 16–256 GB.
• Lower cost than SSDs — common in budget devices.

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External hard drives, optical drives, flash drives, memory cards, NAS

External Hard Drives (SSD, HDD)

• Various sizes and capacities — used for backup, file transfer, or storage.
• Same basic functionality as internal HDDs/SSDs.

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Optical Drives

• Reads/writes data on optical discs using laser technology.

Aspect	Detail
Storage	Data stored as pits (凹点) and lands (平地) on a spiral track (螺旋轨道) of a polycarbonate disc with reflective metal layer
Read	Laser reflects differently off pits/lands; sensors detect light variations → electrical signals
Advantages	High capacity, compatible with older devices
Disadvantages	Slow access, fragile, easily scratched/dusty

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Flash Drives / USB Flash Drives

• Small, portable, connect via USB.
• Uses NAND flash memory.
• Portable, fast transfer, affordable.
• Lower capacity than HDDs/SSDs; data loss possible; losing the device causes security risks.

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Memory Cards 存储卡

• Small, removable storage for portable devices (smartphones, tablets, cameras).
• Uses NAND flash memory.
• Portable, compact (体积小巧), various capacities.
• Easily lost/damaged; may not be compatible (兼容) with all devices.

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Network Attached Storage (NAS)

• Centralized storage accessible by multiple devices on a network.
• Uses HDDs, SSDs, or hybrid (混合使用); can be configured with RAID.
  • RAID (Redundant Array of Independent Disks, 独立磁盘冗余阵列) 把多块硬盘组合在一起，防止硬盘故障并保证数据可用性。
• Centralized, shared access, scalable, high administrative control.
• More expensive than external drives; requires configuration and network connection.