Backup a SQL Server DB log. The SQL Server account (found by looking at the Windows Services window) needs access to the destination directory. If trying to shrink the log, the log must be backed up twice and then then db shrink command will also shrink the log. FYI, shrinking a db can mess up your indexes.
BACKUP LOG MyDb
TO DISK = 'E:\DB Backup\MyDbLog.TRN'
GO