An NDF file is a user defined secondary database file of Microsoft SQL Server with an extension. NDF file is a secondary data files. NDF files are typically stored on a different hard disk or partition than the primary database file. The recommended file name extension for secondary data files is. You have a database with a datafile(PRIMARY) in a filegroup(PRIMARY) which stores metadata as well as user data. If storage space of primary data file is not enough, a new file can be created that adds up data in continuation to it.