You could use any of the following terms: Non-volatile storage, persistent storage, mass storage, non-volatile memory. You could say ‘storage media’ itself. That is very vague.
Auxiliary storage is also a good answer. It is a non-volatile memory (does not lose stored data when the device is powered down) that is not directly accessible by the CPU, because it is not accessed via the input/output channels (it is an external device).
