This backup is a file backup of read-write data from a database that uses the simple recovery model. This is only appropriate ...

This backup is a file backup of read-write data from a database that uses the simple recovery model. This is only appropriate if you plan to set the filegroup to read-only followed by a differential file backup. Consult Books Online for more information on managing read-only data for the simple recovery model. In particular, consider how partial backups are used.