[Element name] DiskName [Description] 
Optional. If the disk that is being added is already registered in the subscription, this element is used to identify the disk to add. If a new disk and the associated VHD are being created by Azure, this element is not used and Azure assigns a unique name that is a combination of the deployment name, role name, and identifying number. The name of the disk must contain only alphanumeric characters, underscores, periods, or dashes. The name must not be longer than 256 characters. The name must not end with period or dash. You can use List Disks to find available disks.