Secondary Storage
Why Computers Need Secondary Storage
When you switch a computer off, the working memory it was using empties completely. That memory is volatile, which means it only holds data while power is flowing. If a computer had nothing else, every file, photo and program would vanish the moment you unplugged it. This is why every computer also has secondary storage — storage that is non-volatile, so it keeps its contents safely even with the power off.
Secondary storage does two big jobs. First, it stores everything permanently: the operating system, your applications, and all your saved documents, music and pictures. Second, it gives a computer a huge amount of room. Working memory is fast but small and expensive, so it is measured in a few gigabytes. Secondary storage is much larger and far cheaper for each gigabyte, which is why a laptop can hold thousands of files while only running a handful of them at once.
Secondary storage is non-volatile: it keeps its data without power. That is exactly why it is used to store the operating system, programs and files permanently — unlike volatile working memory, which loses everything when the power is removed.
Three Families of Secondary Storage
There are three main types of secondary storage, and each one records data in a completely different way. Magnetic storage uses magnetism on spinning surfaces, optical storage uses a laser and a shiny disc, and solid-state storage uses electronic memory chips with no moving parts. Because they work differently, each type is good at some things and weak at others, so choosing the right one depends on the job.
Magnetic Storage
Magnetic storage is the technology inside a traditional hard disk drive (HDD). Data is stored as tiny magnetised patterns on the surface of fast-spinning metal platters. A read/write head floats just above each platter and moves across it to reach the right spot; it detects the magnetism to read data, or changes it to write new data. Every magnetised area represents a binary 1 or 0.
Because the platters spin and the head moves, magnetic drives have moving parts. This makes them slower than solid-state storage and easier to damage if the computer is dropped or knocked. Their great strength is capacity for the price: a single hard disk drive can hold several terabytes and costs very little per gigabyte, which is why they are still common in desktop computers and servers that need to store a lot cheaply.
A second form of magnetic storage is magnetic tape. Tape holds an enormous amount of data at a very low cost, but it is sequential: to reach a particular file the drive must wind through the tape in order, rather than jumping straight to it. That makes tape too slow for everyday use, but ideal for backing up and archiving large amounts of data that are rarely read back.
Optical Storage
Optical storage covers CDs, DVDs and Blu-ray discs. Data is stored along a spiral track as microscopic pits and lands — tiny bumps and flat areas on the disc’s reflective surface. A laser shines onto the track, and the way the light reflects back tells the drive whether it is reading a 1 or a 0.
Optical discs are cheap to produce and easy to carry, so they were widely used to hand out films, music and software. Their weakness is capacity: a CD holds only about 700 megabytes, a single-layer DVD about 4.7 gigabytes, and a single-layer Blu-ray disc about 25 gigabytes. Discs are also fairly slow to read and are easily scratched, which can make the data unreadable.
The optical capacity ladder rises steadily: a CD holds roughly 700 MB, a single-layer DVD about 4.7 GB, and a single-layer Blu-ray disc around 25 GB. Newer formats pack more data into the same size of disc.
Solid-State Storage
Solid-state storage stores data electronically inside flash memory chips. It has no moving parts at all — there is nothing to spin and no head to move. This technology is found in solid-state drives (SSDs), USB flash drives and the memory cards used in cameras and phones.
Having no moving parts brings real advantages. Solid-state storage is the fastest type of secondary storage, it is much more durable because it can survive knocks and drops, and it is silent and uses little power. Those qualities make it perfect for laptops, phones and portable drives. The trade-offs are that it costs more per gigabyte than a magnetic drive, and each memory cell can only be rewritten a limited number of times before it wears out.
An SSD has no moving parts, so it is both faster and more durable than a magnetic hard disk drive. Its main drawbacks are a higher cost per gigabyte and a limited number of times each cell can be rewritten.
Weighing Up the Trade-offs
No type of storage wins on everything, so devices are compared on six characteristics: capacity (how much it holds), speed (how quickly data is read or written), portability (how easy it is to carry), durability (how well it survives damage), reliability (how likely it is to keep working without losing data), and cost (usually measured per gigabyte). The best choice is always a balance of these.
The table below sums up how the three families compare across the main characteristics.
| Characteristic | Magnetic (HDD/tape) | Optical (CD/DVD/Blu-ray) | Solid-state (SSD/flash) |
|---|---|---|---|
| Capacity | Very high (terabytes) | Low (700 MB – 25 GB) | High, but pricier |
| Speed | Medium | Slow | Fastest |
| Portability | Tape and discs portable; internal drives less so | Very portable | Very portable |
| Durability | Low — moving parts | Low — scratches easily | High — no moving parts |
| Cost per GB | Cheapest | Cheap per disc | Most expensive |
Reliability is worth a closer look. A magnetic drive can fail suddenly if its moving parts break, especially after a knock. Optical discs degrade over time and stop working once scratched. Solid-state storage has no parts to break, so it is generally very reliable, but remember that its cells wear out after a great many writes — so no single type is perfectly dependable, and important data should always be backed up.
An everyday laptop: a solid-state drive is the best fit. It is fast, it survives being carried around, it is silent and it saves battery — speed and durability matter more here than the extra cost.
Backing up a large amount of data cheaply: magnetic tape or a high-capacity hard disk drive wins. Both offer enormous capacity for a very low cost per gigabyte, and slow access does not matter for backups that are rarely read.
Handing out a film or software to many people: an optical disc is ideal. Discs are cheap to produce in bulk and easy to post or carry, and the small capacity is enough for the files.
Moving files between two computers: a USB flash drive — a small solid-state device — is convenient, portable and quick for the modest amounts of data involved.