Get Raw Drive Size - powershell

Question

I need to gather volume capacity sizes for all fixed drives on a server. I am limited to using PowerShell 2 because that is what the sysadmins have deployed on the servers. I can get the information on the formatted drives very easily, using the Get-WmiObject Win32_Volume function.
$ComputerName = "."
Get-WmiObject -ComputerName "$computername" Win32_Volume -Filter "DriveType=3" | foreach {
New-Object PSObject -Property #{
SystemName = $_.SystemName
UsageDate = $((Get-Date).ToString("yyyy-MM-dd HH:mm:ss"))
Label = $_.Label
VolumeName = $_.Name
FileSystem = if ($_.FileSystem -eq $null) {"RAW"} else {$_.FileSystem}
Size = if ($_.FileSystem -eq $null) {
Show-Capacity
} else {
$([Math]::Round(($_.Capacity/1GB), 2))
}
Free = if ($_.FileSystem -eq $null) {
"0.00"
} else {
$([Math]::Round(($_.FreeSpace/1GB), 2))
}
PercentFree = if ($_.Capacity -gt 0) {
$([Math]::Round((([float]$_.FreeSpace/[float]$_.Capacity) * 100), 2))
} else {
'0.00'
}
}
}
The Show-Capacity function is as follows:
function Show-Capacity {
$drive=$_.Name
$dpscript = #"
Select Volume '$drive'
Detail Volume
"#
$Temp= #()
[array]$Temp += ,$dpscript | diskpart
foreach ($line in $Temp) {
if ($line.StartsWith("Volume Capacity") ) {
$Values = $line.Split(":")
}
}
$Capacity = $Values.Trim("G", "T", "M", "B", " ")
$Capacity[1]
}
The formatted drives show up correctly as follows:
UsageDate : 2017-11-09 08:08:17
SystemName : MyServerName
Label :
VolumeName : C:\
Size : 99.9
Free : 77.04
PercentFree : 77.11
The Raw Drives show up as follows:
UsageDate : 2017-11-09 08:08:15
SystemName : MyServerName
Label :
VolumeName : I:\
Size :
Free : 0.00
PercentFree : 0.00
I want Raw Drives to show up this way:
UsageDate : 2017-11-09 08:08:15
SystemName : MyServerName
Label :
VolumeName : I:\
Size : 12000.0
Free : 0.00
PercentFree : 0.00
In the PowerShell logs I see the error: "Cannot index into a null array. At {path}:12 char 11. I'm also getting a trim error, but that is of lesser concern. I've been working on this and reading lots of posts for about 2 weeks now and have not been successful.
I tried the method outlined in this line of posts:
https://blogs.technet.microsoft.com/heyscriptingguy/2013/08/30/automating-diskpart-with-windows-powershell-part-5/
This will always be run on local computer, but will be exported to CSV and aggregated.