To not allow the selection of a local folder when selecting an external backup repository.
When performing a backup or scheduled backup in Plesk 11 to an external FTP repository, you can define the backup destination as a local file within the folder structure you are backing up (as in you are using the external FTP repository to back up locally). This causes a backup file to be generated that is many GB in size. The domain and/or subscription then gets suspended. There should be a check in place to not allow backups have a destination within the file structure that is being backed up.
Plesk 12.0 – server backup settings