Storage is non-volatile disk space, and is sometimes called auxiliary memory, secondary memory or external memory. Storage devices, like an external hard drive are used to store files like photos, program files and any other information that needs to kept long term. The data on a storage drive needs to be loaded into RAM before it can be used.

Internal storage

Most computers have internal storage space.

The internal storage drive stores the operating system and software applications. It is normally a hard disk drive (HDD) or a solid state drive (SSD).

External storage

External HDDs and SSDs can also be used to store data. USB memory sticks, memory cards, CDs and DVDs are convenient external storage media. They are normally connected to a computer using USB or FireWire connections.

Hard disk drive (HDD)

Hard drives can store large amounts of data - 200 GB to 1 TB is common in desktop computers.

A magnetic disk drive consists of a number of platters (disks) coated in a magnetic material. They spin at around 7200 rpm. The data is encoded into bits and written onto the surface as a series of changes in the direction of the magnetisation. The data is read by detecting the changes in direction.

Flash memory and solid state drives SSDs

Flash memory is a type of non-volatile ROM memory called EEPROM (electronically erasable programmable read only memory). This is a form of static RAM.

USB memory sticks and memory cards are compact forms of flash memory used with cameras, smartphones and tablet computers. The internal storage of a computer can use a solid state drive (SSD), which is a much larger piece of flash memory.

Flash memory is solid. There are no moving parts, unlike the spinning disc inside a magnetic drive. A SSD uses microscopic electronic switches to store data.

Millions of transistors are either on or off and remain so as long as electricity is applied to the system. SSDs use switches called floating gate transistors. These create a special cage (the gate) that traps the electron flow within it. This gate effect continues whether there is electricity in the system or not, and so storage remains when the power is removed.

SSDs vs HDDs

This table sets out some of the advantages and disadvantages of solid state drives (SSDs) and hard disk drives (HDDs):

CriteriaDifference between SSD and HDD
PriceSSDs are about 10 times more expensive than HDDs per MB
PowerHDDs uses about 3 times more battery power than SSDs
CapacitySSDs are generally no larger than 512 GB but HDDs go up to 4 TB
SpeedSSDs are almost twice as fast at loading up and copying files
NoiseSSDs are silent whereas HDDs make spinning disc sounds
ReliabilityBoth are highly reliable forms of storage but the magnetic disc is more vulnerable to knocks as it has moving parts