Systems and methods for migrating stored backup data between magnetic disks (e.g., from an existing magnetic disk to another disk), such as a new or different magnetic disk in a magnetic storage library, without interrupting or otherwise affecting secondary copy operations (e.g., operations currently writing data to the storage library) utilizing the magnetic storage library, are described. In some embodiments, the systems and methods mark one or more mount paths as full when a running secondary copy operation associated with the mount path has completed a job (regardless of the actual current capacity or intended use of the mount path), and migrate each of the one or more data volumes to a second magnetic disk of the magnetic library when the mount path associated with the data volume is marked as full.