title: Terraform - Azure: Attach Existing Disk - Changing property 'dataDisk.name' is not allowed
body: <p><strong>Environment:</strong></p>
  
  <pre><code>Terraform v0.11.0
  + provider.azurerm v0.3.3
  + provider.tls v1.0.1
  </code></pre>
  
  <p><strong>Problem:</strong></p>
  
  <p>Trying to attach an existing disk on VM being provisioned, getting error as such:</p>
  
  <pre><code>* azurerm_virtual_machine.kafka_vm: compute.VirtualMachinesClient#CreateOrUpdate: Failure responding to request: StatusCode=409 -- Original Error: autorest/azure: Service returned an error. Status=409 Code="PropertyChangeNotAllowed" Message="Changing property 'dataDisk.name' is not allowed."
  </code></pre>
  
  <p>Here is <code>vm.tf</code>:</p>
  
  <pre><code>storage_data_disk {
      name            = "${var.env_short_name}-zookeeper-data01"
      managed_disk_id = "${var.kafka_data_image_id}"
      create_option   = "Attach"
      disk_size_gb  = "30"
      lun             = 5
    }
  </code></pre>
  
  <p>..where <code>managed_disk_id</code> is a variable (i.e., <code>$var.kafka_data_image_id</code>) defined with a full path to the existing disk. Example:</p>
  
  <pre><code>kafka_data_image_id  = "/subscriptions/XXXXX-fcf3-4ec6-b598-XXXXXXXXXXXX/resourceGroups/TEST-Environment-test0001-rg/providers/Microsoft.Compute/disks/test1-kafka-data01"
  </code></pre>
  
  <p>I tried to provide <code>""</code> for the name, but it errored out telling me that I must provide name. Any pointers on what am I doing wrong?</p>
  
  <p>Thanks!!</p>