Test Disk and Photo Rec

 TestDisk digs through a library card catalog for an ignored catalog entry and restores the entry. This works on partitions alike. PhotoRec checks all sectors on a drive for files. You can specify the entire disk or just unallocated space (space without catalogues) for search requests. Photorec is a broad and last ditch check if other methods fail. It is time consuming. Testdisk allows you to systematically restore available pointers (partitions, catalogs) and can undelete files if those pointers are still available.