Secondary Storage

Secondary storage is storage that keeps its contents when the power is switched off, which makes it non-volatile. It holds the operating system, your programs and your files for the long term, so everything is still there the next time you switch the computer on. A smartwatch's built-in flash memory and an external hard drive plugged into a desktop computer are both examples of secondary storage.

Why computers need it

Primary storage has a small capacity compared with secondary storage, and RAM is volatile: switch off the power and everything in RAM disappears. RAM is also expensive, so it could never store an operating system, several open programs and a folder full of videos all at once. Secondary storage solves these problems, because it keeps data without power and gives you far more room at a much lower cost per gigabyte.

The CPU still cannot run a program straight from secondary storage. When you open a program, its instructions and the file you are working on are copied into RAM, and the CPU fetches them from there. That is why a large program takes a moment to start: the copy has to happen first.

How it compares with primary storage

Internal and removable

Secondary storage can be internal, fixed inside the computer, or external and removable, such as a microSD card you slot into a portable speaker to play stored music. Removable storage is handy for backing up files and for taking them to a different place. Backup is only one use, though: the operating system and your programs sit on secondary storage permanently, not just as a spare copy.