NDF files are secondary database files that are used by Microsoft SQL Server. The NDF file is created when the primary .MDF database becomes too big (such as when there is no disk space left). The NDF file may also be manually added to the SQL Server File Group to allow for distribution of data between the two files. NDF files are typically stored on a different hard disk or partition than the primary database file. The NDF file should use the same filename as the corresponding MDF file.