secondary storage; secondary memory
Computing term for non-volatile storage such as hard disks and SSDs, as opposed to main memory (RAM).
Secondary storage includes hard disks and SSDs.
主記憶装置 is main memory (primary storage), typically volatile RAM, while 二次記憶装置 is secondary storage, non-volatile and larger capacity.
Compound of 二次 (secondary) + 記憶 (storage/memory) + 装置 (device). A straightforward technical term.