sqlserver 2008r2 backup to the UNC share directory

I was trying to backup the sqlserver 2008 R2 to the share directory in the LAN,but lots of error trap me in.
I always got the error message as below:
backup database testdb to “\”

BackupDiskFile::CreateMedia: backup device ‘z:backupedm.bak’ can not be created。operation system error 3(can not find the path)

I think there must be some difference between the local disk and the UNC mapping disk.

the priviledge in the UNC share disk is ok,I can exclude it’s problem.
while it seem that it is because the account to start the SQL Server instance,that’s the root cause.
AS SQL Server services were startup by “NT AUTHORITYSYSTEM” which has no right to visit the remote resources in the LAN.If you use the domain user or any local os account who has the priviledge to visit remote resources,the problem will be overcome.
Finally,I change the startup account to administrator,this problem was solved.

Besides,it’s not recommended that you use the local administrator account to boot the SQL Server service,because when the OS admin change the password of administrator account,the SQL Server will can not be startup as the password was incorrect..