Update-AzureVM arm error

Hello,

I use Resource Manage framework. I want to add existing data disk to existing VM. VM is Stopped.

Switch-AzureMode AzureResourceManager
$vm = Get-AzureVM -ResourceGroupName 'myrg' -Name 'myvm'

Add-AzureVMDataDisk -VM $vm `
    -Name 'mydatadisk' `
    -SourceImageUri 'https://myacc.blob.core.windows.net/vhds/mydatadisk.vhd' `
    -VhdUri 'https://myacc.blob.core.windows.net/vhds/mydatadisk.vhd' `
    -CreateOption attach `
    -Caching ReadOnly `
    -DiskSizeInGB $null `
    -Lun 0

Update-AzureVM -ResourceGroupName 'myrg' -Name 'myvm' -VM $vm

I get an error:

Update-AzureVM : A parameter cannot be found that matches parameter name 'Name'.

$vm | Update-AzureVM

I get another error:

Update-AzureVM : PropertyChangeNotAllowed: Changing property 'windowsConfiguration.additionalUnattendContent' is not allowed.
OperationID : 'dcb83bc8-4077-40b2-97fa-38cad5bfacf2'
At line:1 char:7+ $vm | Update-AzureVM+       ~~~~~~~~~~~~~~+ CategoryInfo          : CloseError: (:) [Update-AzureVM], ComputeCloudException+ FullyQualifiedErrorId : Microsoft.Azure.Commands.Compute.UpdateAzureVMCommand

How I can add an existing data disk?

Thank you in advance,

Vadim.