Put() sometimes throws exception when trying to assign drive letter to partition - windows

Question

I have a simple script that assigns a drive letter to any unlettered partition, like the following:
function GetNextAvailableLetter
{
#returns an unused char for drive letter assignment, or $null if none are available
}
foreach ($disk in ( get-wmiobject -class win32_volume | where-object { $_.DriveLetter -eq $null } ) )
{
$letter = GetNextAvailableLetter
if ( $letter -ne $null )
{
$disk.DriveLetter = $letter + ":"
$disk.Put()
}
}
Oddly, sometimes it'll work, and sometimes Put() throws an exception:
Exception calling "Put" with "0" argument(s): "Not supported"
I have no idea why Put() would throw.

Answer 1

I made a couple of empty, driveletterless drives on my computer and was able to recreate this and one other error that I think you might have neglected to mention.
Property 'DriveLetter' cannot be found on this object; make sure it exists and is settable.
At line:2 char:1
+ $disk.DriveLetter = "Q:"
+ ~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : PropertyAssignmentException
Exception calling "Put" with "0" argument(s): "Access is denied.
"
At line:3 char:1
+ $disk.Put()
+ ~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [], MethodInvocationException
+ FullyQualifiedErrorId : DotNetMethodException
The way to solve this would be run the variable $disk though for each loop or something along those lines. Another approach would be to check the $disk.Count ahead of time.
$disk = get-wmiobject -class win32_volume | where-object { $_.DriveLetter -eq $null }
If (($disk) -and ($disk.Count -eq 1)){
$disk.DriveLetter = "Q:"
$disk.Put()
}
The If should in theory protect you from errors when $disk is empty or returns more that one object.

Answer 2

According to the Scripting Guys:
The reason for this error is that the Windows PowerShell prompt is not running with Administrator rights. Unfortunately, the error that bubbles back up from WMI does not tell us that the problem is related to rights.