offline storage
Computing term for storage that is not continuously connected to the system and must be mounted or attached when needed.
The backup is stored on an offline storage device.
オンライン記憶装置 is online storage that is always connected and accessible, while オフライン記憶装置 requires manual mounting or connection.
Compound of オフライン (offline) + 記憶装置 (storage device), a direct calque from English 'offline storage'.