Find Drives with Low Disk Space for all SQL Server Instances using PowerShell

CREATE TABLE Inventory 
(
   [Environment]  nvarchar(128) NOT NULL,
   [ServerName]   nvarchar(128) NOT NULL,
   [InstanceName] nvarchar(128) NOT NULL
)

INSERT INTO Inventory values ('Bad','NoServer','NoServer')
INSERT INTO Inventory values ('Production','ServerA','ServerA')
INSERT INTO Inventory values ('Test','ServerB','ServerB\Test1')

$ErrorActionPreference = "Stop" #stop when an error is encountered
# Declare variables
$server = "ServerC"
$database = "master"
$query = @"
SELECT [Environment], [ServerName], [InstanceName]
  FROM [Inventory]
"@
$threshold = 20 #less than 20% is reported
$SleepTimer = 1000 #after X milliseconds, check if the jobs have finished. 1000 is every second.
$MaxResultTime = 300 #after X seconds, all jobs are killed. 300 is 5 minutes.
$Maxthreads = 20 #number of parallel jobs
# import modules
Import-Module SqlPs -DisableNameChecking
$error.clear() #clear error generated by last command
# get list of instances
$objects = @(Invoke-Sqlcmd -ServerInstance $server -Database $database -Query $query)
#environment setup
$output = @()
$errors = ""
$Jobs = @()
$ISS = [system.management.automation.runspaces.initialsessionstate]::CreateDefault()
$RunspacePool = [runspacefactory]::CreateRunspacePool(1, $MaxThreads, $ISS, $Host)
$RunspacePool.Open()
$script = {
   Param ([string]$environment, [string]$serverName, [string]$instanceName, [int]$threshold)
   $ErrorActionPreference = "Stop"
   $query = "
      SET NOCOUNT ON --No 'rows returned' message
      IF @@VERSION LIKE 'Microsoft SQL Server  2000%'
      BEGIN
         SELECT DISTINCT LEFT([filename], CHARINDEX('\', [filename]) - 1) FROM [dbo].[sysaltfiles]
      END
      ELSE
      BEGIN
         SELECT DISTINCT LEFT([physical_name], CHARINDEX('\', [physical_name]) - 1) FROM [sys].[master_files]
      END"
   try {
      $disks = sqlcmd -S $instanceName -Q $query -h-1 -W -b -r1 2>&1 #no header, no whitespaces, break on error, errors to output, stderr to stdout
      if (!$disks -or $disks.length -eq 0) { return $instanceName + ": Unable to query server" | Out-String } #server may not exist anymore
      $diskList = "" #filter volumes based on drive letter
      foreach ($row in $disks) {
         if ($diskList -eq "") { $diskList = "DriveLetter='$($row)'" }
         else { $diskList += " OR DriveLetter='$($row)'" }
      }
      $result = Get-WmiObject Win32_Volume -ComputerName $serverName -filter "$diskList" | select `
         @{Name="Environment"; Expression={$environment}}, `
         @{Name="InstanceName"; Expression={$instanceName}}, `
         DriveLetter, `
         Label, `
         @{Name="Capacity"; Expression={[math]::Round($_.Capacity/1GB, 2)}}, `
         @{Name="FreeSpace"; Expression={[math]::Round($_.FreeSpace/1GB, 2)}}, `
         @{Name="PercentFree"; Expression={[math]::Round(($_.FreeSpace * 100 / $_.Capacity), 2)}}
      "" #indicate there was no error
      $result | where {$_.PercentFree -lt $threshold} #return results lower than threshold
   }
   catch {
      return $instanceName + ": " + $LastExitCode + " " + $_ | foreach { $_.ToString() } | Out-String #Get-WmiObject error, maybe permissions
   }
}
function CreateThread() {
   param ([string]$environment, [string]$serverName, [string]$instanceName, [ref]$Jobs)
   $PowershellThread = [powershell]::Create().AddScript($script) #scriptToRun
   $PowershellThread.AddArgument($environment) | out-null
   $PowershellThread.AddArgument($serverName) | out-null
   $PowershellThread.AddArgument($instanceName) | out-null
   $PowershellThread.AddArgument($threshold) | out-null
   $PowershellThread.RunspacePool = $RunspacePool
   $Handle = $PowershellThread.BeginInvoke()
   $Job = "" | select Handle, Thread, object
   $Job.Handle = $Handle; $Job.Thread = $PowershellThread
   $Jobs.value += $Job
}
$ResultTimer = Get-Date #start time
#start processing first task for each instance
for ($i=0; $i -lt $objects.length; $i++) {
   CreateThread $objects[$i]."Environment" $objects[$i]."ServerName" $objects[$i]."InstanceName" ([ref]$Jobs)
}
while (@($Jobs | where {$_.Handle -ne $Null}).count -gt 0) {
   #update completed jobs, get errors and result, and dispose them
   foreach ($Job in @($Jobs | where {$_.Handle -ne $Null -and $_.Handle.IsCompleted -eq $True})) {
      $results = $Job.Thread.EndInvoke($Job.Handle)
      if ($results[0] -and $results[0] -ne "") {
         $errors += $results[0]
      }
      $output += $results[1]
      #end thread
      $Job.Thread.Dispose()
      $Job.Thread = $Null
      $Job.Handle = $Null
   }
   #show progress
   $inProgress = @($Jobs | where {$_.Handle.IsCompleted -eq $False}).count
   Write-Progress `
      -Activity "Gathering data" `
      -PercentComplete (($objects.length - $inProgress) * 100 / $objects.length) `
      -Status "$inProgress pending"
   #exit on timeout
   $currentTime = Get-Date
   if (($currentTime - $ResultTimer).totalseconds -gt $MaxResultTime) {
      Write-Error "Child script appears to be frozen, try increasing MaxResultTime"
      break
   }
   #sleep
   Start-Sleep -Milliseconds $SleepTimer
}
#dispose thread pools
$RunspacePool.Close() | Out-Null
$RunspacePool.Dispose() | Out-Null
$errors #return errors
$output | Format-Table -Auto #return results