Not sure why SystemName would be null. Let's try forcing the systemname. Replace the existing Get-Vol function with this version:

#######################

function Get-Vol

{

param($computerName)

Get-WmiObject -computername "$ComputerName" Win32_Volume -filter "DriveType=3" |

foreach { add-member -in $_ -membertype noteproperty UsageDT $((Get-Date).ToString("yyyy-MM-dd"))

add-member -in $_ -membertype noteproperty SizeGB $([math]::round(($_.Capacity/1GB),2))

add-member -in $_ -membertype noteproperty FreeGB $([math]::round(($_.FreeSpace/1GB),2))

add-member -in $_ -membertype noteproperty PercentFree $([math]::round((([float]$_.FreeSpace/[float]$_.Capacity) * 100),2))

add-member -in $_ -MemberType noteproperty SystemName $ComputerName -force -PassThru } |

select UsageDT, SystemName, Name, Label, SizeGB, FreeGB, PercentFree

}# Get-Vol