Differential partial backups work like differential database backups but are focused on the same type of data as partial backups. The extents that have changed in filegroups that are not read-only are captured in this type of backup. This includes the primary filegroup and any read/write filegroups defined at the time of the backup. Like differential database backups, these backups also require a differential base, but it must be a single differential base. In other words, multiple base backups that have been taken at different times for different database files will not work. You must use a single base backup that encompasses all of the database files.