You may have seen this error Error: Object SharePoint_Config failed in event OnBackup. For more information, see the error log located in the backup directory. SqlException: Cannot open backup device '\\servername\backup\spbr0001\0000001.bak'. Operating system error 5(error not found). After reading this article http://technet2.microsoft.com/Office/en-us/library/b4dfe06a-40a5-4826-8d4b-

Read the complete post at http://dhunter-thinkingoutaloud.blogspot.com/2007/07/sharepoint-2007-backup-failed.html