I used a mapped network drive from File Server as drive F: on SQL Server machine, then added that mapped drive F: as Backup network device using stored procedure sp_addumpdevice. After that, from Enterprise Manager I tried to backup database Master using backup device 'F':.
I've got error message MS SQL-DMO (ODBC SQL State: 42000) - Cannot open backup device 'asnetwork'. Device error or device off-line. See the SQL Server error log for more details.
BACKUP DATABASE is terminating abnormaly.
In error log I've found next:
BackupDiskFile::Create Media:Backup device 'F:\asbckp.bak' failed to create. Operating system error = 5 (error not fpund).
I've also tried to manage without network mappings, using simple file path to the network share on File Server as a backup device, but with similar results.