A computer needs a way to store the data as you work on it. This is performed automatically by the computer system and is called primary storage. It is the only memory the CPU can access directly. Primary storage comes in two varieties: RAM (random-access memory), which is lost when the power is turned off and ROM (read-only memory). 

SSD Drives

The second type of memory is called secondary storage and is sometimes called external memory. Hard disk drives (HDD) and solid state drives (SSD) are both commonly used for secondary storage. Secondary storage saves files and program permanently, even when the power is turned off. Hopefully you already understand the importance of saving files to a hard disk while you are working on them before you turn your computer off. Average internal SSD hard drives have a capacity of around 500 GB to 1 TB and are fast and reliable but more expensive than an internal HDD, which typically have larger storage capacity than SSD.

Types of storage devices