Differential Backups and New Files
This topic is relevant for all types of databases.
A differential backup might include files that have not been backed up before. For example, if a file is added to a database after a full database backup, the next differential database backup includes the new file. This allows for the database to be backed up conveniently without having to know about individual files.
Generally, files that have no differential base are included in a differential backup if any one of the following is true:
The backup is a differential database backup.
The backup is a differential partial backup, and the new file is a member of a read/write filegroup.
The file is specified in the BACKUP statement.
The filegroup containing the file is listed in the BACKUP statement.
For information about how new files are handled at restore time, see Controlling Creation of Added Files During Roll Forward.