Resizing a VM's Windows system OS with Set-HardDisk

With vSphere PowerCLI Update 1 there is now an automated method of increasing the system disk of a virtual machine. This essentially performs the same thing as I’ve been doing manually for a while now - the disk is resized and mounted to a ‘helper vm’ and diskpart extends the disk, the helper VM is then shutdown and the risk removed, and when the resized VM starts up the OS prompts for a final reboot after detecting the disk change.

Note that both the target and helper VM must be powered off, and credentials must be passed or the helpervm argument is ignored. The guest credentials are used on the Windows helper VM to authenticate using the VMtools mechanism to invoke a command inside a VM.

$guetsusername = "domain\username"
$guestpass = Read-Host "Guest password (administrator of the server)?" -assecurestring
$hostusername = "root"
$hostpass = Read-Host "Host Password (VI SDK access to the ESX host)?" -assecurestring
$hostcredential = new-object System.Management.Automation.PSCredential($hostusername, $hostpass)
$guestcredential = new-object System.Management.Automation.PSCredential($guetsusername, $guestpass)
$sizeGB = 30
$sizeKB = $sizeGB *1024*1024
$harddisk = Get-HardDisk -vm vm01
Set-HardDisk -HardDisk $harddisk[0] -capacityKB $sizeKB -hostcredential $hostcredential -guestcredential $guestcredential -helpervm $vm

An error occurred when trying to use domain credentials to pass to the vCenter SDK – host credentials are required, not credentials to vCenter (which seems odd).

Set-HardDisk -HardDisk $harddisk[0] -capacityKB 31457280 -hostcredential $hostcredential -guestcredential $guestcredential -helpervm helpervm
Set-HardDisk : 4/02/2010 1:25:04 PM    Set-HardDisk    919835D1-AD13-41DF-ABDE-D630E96FE722    While performing operation 'Connect to host service 'https://esx01/sdk' at port 902' the following error occured: 'Insufficient permissions in host operating system'
At line:1 char:13
+ Set-HardDisk  <<<< -HardDisk $harddisk[0] -capacityKB 31457280 -hostcredential $hostcredential -guestcredential $guestcredential -helpervm helpervm

Another error occurred when trying to resize a VM, the script used failed.  Note that I tried this manually and it also failed (must have been something particular to that VM).  It was interesting to see that from this it looks like the cmdlet uses the invoke-vmscript to run a remote script through the VMtools interface that calls the diskpart commands.
Set-HardDisk : 4/02/2010 11:22:11 AM    Set-HardDisk    919835D1-AD13-41DF-ABDE-D630E96FE722    Execution of the  script in guest OS on VM 'helpervm' failed: '
Microsoft DiskPart version 5.2.3790.3959
Copyright (C) 1999-2001 Microsoft Corporation.
On computer: HELPERVM
Disk 1 is now the selected disk.
Volume 2 is the selected volume.
The volume you have selected may not be extended.
Please select another volume and try again.
'
At line:1 char:13
+ Set-HardDisk  <<<< -HardDisk $harddisk[0] -capacityKB 31457280 -hostcredential $hostcredential -guestcredential $guestcredential -helpervm $vm

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