Auxiliary storage is any storage that is made available to the system through input/output channels. This term refers to any addressable storage that is not within the system memory (RAM). These storage devices hold data and programs for future use and are considered nonvolatile storage that retains information even when power is not available. They trade slower read/write rates for increased storage capacity.
Auxiliary storage may also be referred to as secondary storage.
Techopedia explains Auxiliary Storage
Auxiliary storage, secondary storage, or external storage are devices that store noncritical system data like documents, multimedia and programs, which are used whenever they are required. These files are invoked from the auxiliary storage when needed and then transferred to the primary storage so that the CPU can process them. The results of the process can also be sent back to the auxiliary storage for later retrieval.
The best example of auxiliary storage is hard disk drives and optical storage media like CDs, DVDs, and Blu-ray. Other auxiliary storage belongs to the peripheral devices category as well, such as flash drives and any type of memory card.