External memory refers to external hard drives, discs and USB thumb drives. These are all media kept externally to your PC case.
External media is also known as auxiliary memory or secondary storage. Hard drives sold with laptops are usually of a comparably low capacity when compared to their desktop counterparts due to size. This means that many people turn to alternative long-term storage devices that complement the internal memory.
Blu-rays, DVDs and CDs are considered external memory even though the drive is often found within the PC case. This is because the discs can be changed without removing PC components.