The need for secondary storage devices
Introduction
Computer systems need storage devices. Without them, you would have to load up any new programs and data files every time you switched on and used a computer. Storage devices are known as 'secondary storage devices'. This is because the computer's Central Processing Unit (CPU) doesn't work directly with the secondary storage devices. CPUs work with 'primary memory', usually referring to RAM. Secondary storage devices can be thought of as suitcases! Their job is to hold lots of applications and data, ready for when they are needed.
Volatile v non-volatile devices
Secondary storage devices, for example, hard drives are 'non-volatile'. That means that when the power is removed from them, for example, when you switch your computer off, they keep their contents.The next time you switch your computer on, the operating system can be loaded into RAM automatically using the hard drive and you can also open any programs you want to use straight away - you don't have to reload them from DVDs or the Internet again. Storage media can be used to hold operating systems, applications and files, amongst other types of software.
RAM on the other hand is volatile. That means when you switch the power off your computer, the contents of RAM disappear. If you didn't have a storage system in your computer, you would have to load up the operating system, any programs you wanted to use and your personal files every time you switched your computer on. This would take a time and wouldn't be very convenient.
Size and cost
Storage media like hard drives are huge. They need to be because the operating system, programs and files like video files can be very large. RAM is relatively small compared to hard drives. They don't hold anywhere near the same amount of programs as a hard drive. In fact, the cost per byte of primary memory like RAM is far higher than the cost per byte of a hard drive.