4.7.4 External Hardware Devices: Secondary Storage

What is secondary storage?

Secondary storage refers to non-volatile storage devices used to store data in a computer system, such as hard disk drives, solid-state drives, optical discs, and USB drives.

Secondary storage refers to non-volatile storage devices used to store data in a computer system indefinitely

What are the key types of secondary storage?

- Hard disk drives

- Solid state drives

- Optical disks

What are the components of a hard disk drive?

- Circular platters made from a magnetic material

- Platters split into concentric tracks

- Actuating arm

- Read/write head

How is data written to/read from a hard disk drive?

- The read/write head changes the polarity of parts of the disk to create a binary pattern

- Changes create an electromagnetic pulse, read as a 1

What is disk latency?

The time taken to read/write disk data

- Seek delay

- Rotational delay

- Transfer time

What are the positives of using a hard disk drive?

- Good read and write speeds (≈ 100 MB/s)

- High capacity (500 GB - 5TB)

How can the capacity of a hard disk drive be increased?

- Adding more platters

- Decreasing the width of tracks

What are the negatives of using a hard disk drive?

- High latency

- Moving parts limits portability

- Bulky

- Heavy

- High power consumption

What are hard-disk drives best suited for?

Desktop PCs and servers

What are the components of a solid-state drive?

- NAND flash memory cells

- Controller

What is NAND flash memory?

- Non-volatile

- Formed of floating gate transistors which store information by trapping electrical charge

How is data stored on a solid-state drive?

Stored in pages which are combined to form blocks

What are the advantages of using a solid-state drive?

- Low latency

- Very high speeds (≈ 500 MB/s)

- Lightweight

- Portable

- Low power consumption

What are the disadvantages of using solid-state drives?

Relatively low capacity (≈ Under 1TB)

What are solid-state drives best suited for?

Laptops, phones and tablets

What are the types of optical disk?

- CD-R: Recordable (Written only once and cannot be erased)

- CD-ROM: Read only

- CD-RW: Rewritable

How do optical disks work?

- One continuous track spirals from the centre of the disk to the outside edge

- Low-power laser beam is passed over the surface

- Disk reflects onto a photodiode

- When incident on a pit, light is scattered in different directions

- Resulting patter can be converted into a digital binary signal

How do recordable and rewritable optical disks work?

Patters of reflections and scatters are created by an opaque dye on the disk’s surface

→ Data is written with a photosensitive dye and high-power laser on recordable disks

What are the advantages of using optical disks?

- Cheap

- Lightweight

- Portable if protectedW

What are the disadvantages of using optical disks?

- Low capacity (Highest: Blu-Ray 25GB)

- Low speeds (≈ 30 MB/s)

- High latency

- Easy to damage

- High power consumption

What are optical disks best suited for?

Sharing and distributing small volumes of data