What do you mean by 'mirror backup'?
If you mean, \BACKUP DATABASE... TO DISK ... MIRROR TO ... then that has nothing whatsoever to do with database mirroring.
If you do mean the MIRROR TO clause of backup database, then all specified locations must exist and be accessible or the backup will fail.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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