Including New Forests in Incremental Backups
Incremental backup supports backup of a forest added since last full backup. If you add a new forest after a full backup of your database, you can include the new forest as part of your next incremental backup.
Attach the new forest to your database. It automatically appears in the list of forests to be backed up in the Confirm backup step (Step 12 in Backing Up a Database Immediately).
Select the forest to include it in the backup and click ok. See Backing Up a Database for more information.