ALTER DATABASE [DB] MODIFY FILEGROUP [A] READ_ONLY
ALTER DATABASE [DB] MODIFY FILEGROUP [B] READ_ONLY

BACKUP DATABASE [DB] READ_WRITE_FILEGROUPS TO DISK = N'C:DB.BAK'

RESTORE DATABASE [DB]
FILEGROUP = 'PRIMARY', FILEGROUP = 'C'
FROM DISK = 'C:DB.BAK'
WITH PARTIAL, RECOVERY