Secondary storage is much slower than primary storage, and cannot be accessed directly by the CPU — its contents must first be transferred to RAM. However, it has much greater capacity so it can store lots more data.
Secondary storage types are:
- Magnetic
- Optical
- Solid state
Magnetic Secondary Storage
Magnetic storage includes hard disk drives and floppy disks (although these are now obsolete).
Data is stored as magnetically charged particles on the disk's surface. Positive and negative charges are interpreted as 1s and 0s which the computer uses as data.
Hard disk drives can have a very high capacity, but are much slower than solid state media.
Pros and cons of magnetic storage
- Hard disk drives are very high capacity
- Faster than optical disks
- Low price per gigabyte
- Slower than solid state
- Precision moving parts make them less durable than other media
- Can be relatively large and heavy, so not suitable for modern portable devices
Optical Secondary Storage
Optical storage includes CDs, DVDs, and Blu-Ray discs.
Data is read from the disc by beaming a laser at the metal layer under the disc's surface.
Optical discs have mostly been replaced by streaming services such as Spotify and Netflix. This is partly due to convenience, but also because optical media has low capacity: a CD stores 670 MB of data, a DVD stores 4.7 GB, and a Blu-ray stores 25 GB. (Capacities stated are for single layer, single sided discs).
Pros and cons of optical storage
- Cheap to manufacture and buy a disc
- Light and portable
- Easily damaged by scratches
- Expensive per gigabyte
- Low capacity
Solid State Secondary Storage
Solid state storage includes solid state disks (SSD), memory sticks, and flash media such as SD cards.
Solid state media stores data in a similar way to RAM, but is non-volatile. This means data is retained even after power is lost. It is very fast, physically small, and uses little power.
Capacity is less than most magnetic media, and it is very expensive per gigabyte.
It has mostly replaced magnetic and optical media.
Pros and cons of solid state storage
- Very fast
- Light, portable, and durable – ideal for use in small, portable devices
- High capacity
- Expensive per gigabyte
- Lower capacity than magnetic storage (but rapidly increasing!)
- Limited number of write-cycles limit its lifespan
Exam tips
You will be expected to be able to compare characteristics of diffent types of storage, including:
- Cost per gigabyte: a memory stick may only cost a few pounds, but it does not store very much so its cost per gigabyte will be high. A hard disk drive is much more expensive, but will store much more data, so its cost per gigabyte will be low.
- Capacity: how much data can it store, and is this a suitable amount for its intended purpose?
- Durability: how easily damaged is it?
- Portability: does its weight and durability make it suitable for use in portable devices?
- Speed: how quickly can it access stored data? Is it fast enough for its intended purpose?
Misconceptions
You may be asked to recommend a type of storage. The examiner will be looking for magnetic, optical, or solid state not a specific media such as hard disk, CD, or SSD.
Should you need to discuss a memory stick, you should not refer to it as a "USB". USB is a type of connector, not a type of storage.