Secondary Storage Devices

• A HDD consists of:

  • Circular platters
  • Actuating arm
  • Read/write head
• Platters are made from a magnetic material.

  On which, data is written in concentric tracks which is then divided into sectors.
• The actuating arm allows the read/write head to change the magnetic polarity of parts of the platter.
• Advantages of HDD:

  • Large capacity
  • Relatively cheap
• Disadvantages of HDD:

  • Many moving parts, so susceptible to damage from movements.
  • Not very fast
• An SSD consists of:

  • NAND flash memory cells
  • Controller
• NAND flash is non-volatile.

  The memory cells are formed of floating gate transistors which trap electrical charge which represents data.
• In SSDs, data is stored in pages, which form blocks
• SSDs cannot overwrite data. Instead, the controller must completely erase the entirety of a page before writing new information.
• Advantages of SSDs:

  • No moving parts
  • Higher read/write speeds
• Disadvantages of SSDs:

  • Expensive
  • Relatively low capacity
• Optical disks are either read-only, recordable or rewritable depending on what the use is.
• Optical disks store data by using pits and lands. This can be converted to digital signals of binary.
• A low powered laser reflects the incident light back onto a photodiode. Pits scatter the laser instead of being reflected unlike lands.
• Advantages of opticals disks:

  • Portable
• Disadvantages of Optical Disks:

  • Slow
  • Low capacity