Microsoft SQL Server 2014 (SP2-CU8) (KB4037356) - 12.0.5557.0 (X64) Oct 3 2017 14:56:10 Copyright (c) Microsoft Corporation Enterprise Edition: Core-based Licensing (64-bit) on Windows NT 6.3 <X64> (Build 9600: ) (Hypervisor)

declare @Path varchar(500), @DBName varchar(128)
--прописать бд
select @DBName = 'BD_W'
--прописать путь лога
select @Path = 'y:\'
declare @FileName varchar(4000)
select @FileName = @Path + @DBName + '_Log_'+ convert(varchar(8),getdate(),112) +'.trn'
backup log @DBName to disk = @FileName

Диск "у" это приаттаченый сетевой диск.

Date 1/10/2018 10:00:02 AM
Log Job History (!log)

Step ID 1
Server SQL-server
Job Name !log
Step Name BD_W
Duration 00:00:00
Sql Severity 16
Sql Message ID 3013
Operator Emailed
Operator Net sent
Operator Paged
Retries Attempted 0

Message
Executed as user: domen\user. The backup data at the end of "y:\BD_W_Log_20180110.trn" is incorrectly formatted. Backup sets on the media might be damaged and unusable. To determine the backup sets on the media, use RESTORE HEADERONLY. To determine the usability of the backup sets, run RESTORE VERIFYONLY. If all of the backup sets are incomplete, reformat the media using BACKUP WITH FORMAT, which destroys all the backup sets. [SQLSTATE 42000] (Error 3266) BACKUP LOG is terminating abnormally. [SQLSTATE 42000] (Error 3013). The step failed.