The formal distinction between online, nearline, and offline storage is:

• Online storage is immediately available for I/O.
• Nearline storage is not immediately available, but can be made online quickly without human intervention.
• Offline storage is not immediately available, and requires some human intervention to become online.

For example, always-on spinning hard disk drives are online storage, while spinning drives that spin down automatically, such as in massive arrays of idle disks (MAID), are nearline storage. Removable media such as tape cartridges that can be automatically loaded, as in tape libraries, are nearline storage, while tape cartridges that must be manually loaded are offline storage.