1.4: Secondary Storage & Data Units
Eduqas / WJEC
Secondary storage (also known as backing storage) is non-volatile storage used to save and store data that can be accessed repeatedly.
Secondary storage is not directly embedded on the motherboard (and possibly even external) and therefore further away from the CPU so it is slower to access then primary storage.
CAPACITY: The maximum amount of data that can be stored.
DURABILITY: The physical strength of the device, to withstand damage.
PORTABILITY: How easy it is to carry the device around.
ACCESS SPEED: How quickly data on the device can be read or edited.
COST: The average price it costs to purchase a storage device.
A magnetic hard disk drive (HDD) is the most common form of secondary storage within desktop computers. A read/write head moves nanometres above the disk platter and uses the magnetic field of the platter to read or edit data.
An obsolete (no longer used) type of magnetic storage is a floppy disk but these have been replaced by solid state devices such as USB sticks which are much faster and have a much higher capacity.
Another type of magnetic storage that is still used is magnetic tape. Magnetic tape has a high storage capacity but data has to be accessed in order (serial access) so it is generally only used by companies to back up or archive large amounts of data.
Optical storage uses a laser to project beams of light onto a spinning disc, allowing it to read data from a CD, DVD or Blu-Ray.
This makes optical storage the slowest of the four types of secondary storage.
Disc drives are traditionally internal but external disc drives can be bought for devices like laptops.
Magnetic Storage Characteristics:
✓ - Large CAPACITY and cheaper COST per gigabyte than solid state.
X - Not DURABLE and not very PORTABLE when powered on because moving it can damage the device.
X - Slow ACCESS SPEED but faster than optical storage.
Optical Storage Characteristics:
X - Low CAPACITY: 700 MB (CD), 4.7 GB (DVD), 25 GB (Blu-ray).
X - Not DURABLE because discs are very fragile and can break or scratch easily.
✓ - Discs are thin and very PORTABLE. Also very cheap to buy in bulk.
X - Optical discs have the Slowest ACCESS SPEED.
Magnetic Disks are spelled with a k and Optical Discs have a c.
Solid State Storage
There are no moving parts in solid state storage.
SSDs (Solid State Drives) are replacing magnetic HDDs (Hard DIsk Drives) in modern computers and video game consoles because they are generally quieter, faster and use less power.
A USB flash drive (USB stick) is another type of solid state storage that is used to transport files easily because of its small size.
Memory cards, like the SD card in a digital camera or a Micro SD card in a smartphone, are another example of solid state storage.
When you store data in 'the cloud', using services such as Google Drive or Dropbox, your data is stored on large servers owned by the hosting company. The hosting company (such as Google) is responsible for keeping the servers running and making your data accessible on the internet.
Cloud storage is very convenient as it allows people to work on a file at the same time and it can be accessed from different devices. However, if the internet connection fails, or the servers are attacked then the data could become inaccessible.
Solid State Characteristics:
X - High CAPACITY but more expensive COST per gigabyte than magnetic.
✓ - Usually DURABLE but cheap USB sticks can snap or break.
✓ - The small size of USB sticks and memory cards mean they are very PORTABLE and can fit easily in a bag or pocket.
✓ - Solid State storage has the fastest ACCESS SPEED because they contain no moving parts.
Cloud Storage Characteristics:
✓ - Huge CAPACITY and you can upgrade your subscription if you need more storage.
✓ / X - Cloud storage is difficult to rank in terms of PORTABILITY, DURABILITY and ACCESS SPEED because it depends on your internet connection. A fast connection would mean that cloud storage is very portable (can be accessed on a smartphone or tablet) but a poor connection would make access difficult.
✓ - Cloud storage is typically free for a certain amount of storage. Users can then buy a subscription to cover their needs - Dropbox allows 2 GB for free or 2 TB for £9.99 a month.
Data Storage Units
0 / 1
All data in a computer system is made up of bits.
A single bit is a 0 or a 1.
4 bits (such as 0101 or 1101) is called a nibble.
1,024 bytes is called a kilobyte.
A kilobyte can store a short email.
8 bits is called a byte.
A byte can store a single character.
1,024 kilobytes is called a megabyte.
A megabyte can store about a minute of music.
1,024 megabytes is called a gigabyte.
A gigabyte can store about 500 photos.
1,024 terabytes is called a petabyte.
A petabyte can store about 1.5 million CDs.
1,024 gigabytes is called a terabyte.
A terabyte can store about 500 hours of films.
More data storage units:
1,024 petabytes is called a exabyte.
1,024 exabytes is called a zettabyte.
1,024 zettabytes is called a yottabyte.
1.4 - Secondary Storage:
1. Rank magnetic, optical and solid-state storage in terms of capacity, durability, portability, speed and cost. For example, magnetic has the highest capacity, then solid-state, then optical. This could be completed in a table. 
2. Justify which secondary storage should be used in each scenario and why it is the most appropriate:
a. Sending videos and pictures to family in Australia through the post. 
b. Storing a presentation to take into school. 
c. Storing project files with other members of a group to work on together. 
d. Backing up an old computer with thousands of files to a storage device. 
3. Put the following data storage units in order from smallest to largest:
a. kilobyte - gigabyte - byte - megabyte - nibble - bit 
b. gigabyte - petabyte - kilobyte - exabyte - terabyte - megabyte