If you are going to backup to a mapped network drive, make sure that the FileMaker Server 7 service is using Log On to logon as a domain user on the network. If you check the properties of the service, it runs by default as a local account. This will not allow you to write to a remote volume.
Once you change the logon, restart the service. Then when you create the backup schedule, the path you want will be something like "filewin://server_name/directory/"
That should work for you.