Problem attaching an existing virtual disk with it's content using Vbox provider

I am new to Vagrant. I wanted to mount the an existing virtual disk as a second disk using Virtual box provider. I have tried with the following code but nothing got attached. So how do I attach an existing virtual disk using Vbox provider? Does ‘createhd’ is mandatory before ‘storageattach’ or have I missed something?

shared_disk = "SharedData/data.vmdk"

Vagrant.configure("2") do |config| = ''
  config.vm.box_url = './'
  config.vm.guest = 'windows'
  config.vm.boot_timeout = 10800 "private_network", ip: settings_vm['ip_address'], netmask: ""

  # Make Vagrant use WinRM for remote scripting
  config.vm.communicator = 'winrm'
  config.winrm.username = settings_secrets['win-user']
  config.winrm.password = settings_secrets['win-pass']
  config.winrm.timeout = 10800 # 3 hours
  config.winrm.retry_delay = 60 # seconds
  config.winrm.max_retries = 60 #60 minutes in total
  config.winrm.execution_time_limit = 10800 # 3 hours
  config.vm.provision :"file", :source => "~/tools/windows", :destination => settings_recipe['additional_software_path']

  # Disable synced folder for now - rsync has issues working with sudo_command
  config.vm.synced_folder '.', '/vagrant', disabled: true

  config.vm.provider :vsphere do |vsphere| = settings_global['vsphere_host']
    vsphere.user = settings_secrets['vsphere-user']
    vsphere.password = settings_secrets['vsphere-pass']
    vsphere.insecure = true
    vsphere.wait_for_sysprep = true
    vsphere.ip_address_timeout = 7200
    vsphere.cpu_count = settings_vm['cpu_count']
    vsphere.memory_mb = settings_vm['memory_mb']

    vsphere.template_name = settings_vm['template']
    vsphere.vm_base_path = 'GPD/Core'
    vsphere.data_store_name = 'mydata:datastore2' = settings_vm['vm_name']
    vsphere.customization_spec_name = 'vagrant-windows'
    vsphere.vlan = 'dvPG_Vlan_111_Qa_Server_Vlan'


  # attach storage devices
  config.vm.provider :virtualbox do |vbox|
    vbox.customize ['storageattach', :id, '--storagectl', 'mydata:datastore2', '--port', 1, '--device', 0, '--type', 'hdd', '--medium', shared_disk]


You can use Vagrant to attach disks using the :disk option. Check out the docs: Disks for VirtualBox Provider | Vagrant by HashiCorp

Note that this is an experimental feature.