I have been configuring STSADM backups today on a few SharePoint & WSS WFE servers to backup to a file share. These have been scheduled using the task scheduler and some batch files.
I had read before that the service account running the SQL instance would need modify permissions on the on the file share (both Sharing and Security permissions), and I am sure this is how we have it configured for another server.
However when I executed the backup it kept failing with the error message –
Object objectName failed in event OnBackup. For more information, see the error log located in the backup directory. SqlException: Cannot open backup device Operating system error 5(Access is denied.)
I did some searching around and eventually came across this TechNet thread –
It seems that modify permissions is not enough and that the SQL service account needs full control permissions on the file share. I changed the permissions to full control and hey presto the backup worked!
Hope this helps.