Powershell G-cloud attach disk to instance - powershell

Question

I`m following the documentation:
https://googlecloudplatform.github.io/google-cloud-powershell/#/google-compute-engine/GceInstance/Set-GceInstance
I cant get the following code working:
$disk = Get-GceDisk disk-snapshot-instance-1
Set-GceInstance -Name instance-1 -AttachDisk $disk
When I replace $disk for disk-snapshot-instance-1 I get the same error:
Set-GceInstance : Parameter set cannot be resolved using the specified named parameters.
At line:1 char:1
+ Set-GceInstance -Name instance-1 -AttachDisk $disk
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidArgument: (:) [Set-GceInstance], ParameterBindingException
+ FullyQualifiedErrorId : AmbiguousParameterSet,Google.PowerShell.ComputeEngine.SetGceInstanceCmdlet
The thing I don't understand is that it works for me to remove a disk when attaching it manually through the G-cloud interface.
Set-GceInstance -Name instance-1 -RemoveDisk $disk
My question:
Why can't I attach a disk to an instance with the above code, while removing the disk works?

Answer 1

The correct commands are:
Set-GceInstance $instance -AddDisk $disk1 -Zone $zone
Set-GceInstance $instance -RemoveDisk $disk1 -Zone $zone
I create an instance with 2 disks using gcloud and then:
Get-GceDisk | Format-List -Property Name
This returns:
Name : disk-1
Name : disk-2
Name : instance-1
Then I can:
$zone = "us-west1-c"
$disk2 = Get-GceDisk disk-2
Set-GceInstance instance-1 -RemoveDisk $disk2 -Zone $zone
Set-GceInstance instance-1 -AddDisk $disk2 -Zone $zone
HTH