Migrating data is done to free up disk space. When they were last used. Cost to retrieve the file. The device to which the file is migrated can also depend on these factors. Small files might go to optical disk while large files are sent to a tape robot.
Migration can also occur from one tertiary storage device to another.
Tertiary storage Moving data from tertiary to disk Data is moved from tertiary to disk when it is needed. It may also be prefetched if the system believes that the file might be used soon. Tertiary storage is very much an ad hoc art these days. System designers build their systems based on what others have done because there is relatively little concrete research on what works and what does not. This is very important because a user notices even a single miss.