Param($DriveLetter)
if ($PSBoundParameters.Count -ne 1) { exit }
$DriveLetter = $DriveLetter.Substring(0,1)
try {
  $free = (Get-PSDrive -Name $DriveLetter -ErrorAction Stop).Free
  return ($free / 1024 / 1024 / 1024)
}
catch {
  exit
}