Online restores were new to SQL Server 2005 and continue to be supported in SQL Server 2012. They allow a filegroup, file, or specific page within a file to be restored while the rest of the database is online. The file or filegroup that is being restored to must be offline during the duration of the online restore.
You should take a full backup of a database immediately before taking a read-only file offline. This simplifies the online restore process and eliminates the need to apply a bunch of transaction log backups prior to the online restore. This applies only to databases in full or bulk-logged recovery.
The following example demonstrates how to take a read-only file offline:
ALTER DATABASE ...