SQL Server 2008 Database Restore Error - Cannot open backup device 15105
OS Error code 3 is ERROR_PATH_NOT_FOUND: The system cannot find the path specified. With a path starting with Z: my pshyhic powers tell me it is a mapped drive. Drive mapping is session scoped and I'm convinced your service session does not see Z:. Mapping a drive to be seen by a service is, for all practical purposes, impossible.

Use a valid path. Use UNC names like \\server\share. Consider the security implication of impersonation and delegation when accessing remote shares.