Database Mirroring works on transaction logs. It applies transaction logs from Principle to Mirror. But Operation like addition of file is no log operation because of that file not be added to Mirror automatically.

Steps to Add Database File on Database involved in DB Mirroring :-

1) Break the Mirroring

ALTER DATABASE USERDB1 SET PARTNER OFF

2) Add New Data or log file, whatever is required

USE [master]

GO

ALTER DATABASE [UserDB1] ADD FILE

( NAME = N’UserDB1_1',

FILENAME = N’H:MSSQL12.INS1MSSQLDATAUserDB1_1.ndf’ ,

SIZE = 3072KB , FILEGROWTH = 1024KB )

TO FILEGROUP [PRIMARY]

3) Take Transaction Log backup at Primary

BACKUP LOG USERDB1 TO DISK = ‘C:USERDB1_LOG.TRN’

4) Restore Step 3 Transactional Backup on Mirror with Move option

RESTORE LOG [USERDB1] FROM

DISK = N’C:USERDB1_LOG.TRN’

WITH Move N’UserDB1_1' TO

N’I:MSSQL12.SQL14I2MSSQLDATAUserDB1.ndf’,

NORECOVERY, NOUNLOAD, STATS = 10

5) Reestablished DB Mirroring again

First on Mirror

ALTER DATABASE USERDB1

SET PARTNER = ‘TCP://SQL2014.RohitGarg.local.in:5022’