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The Get-EsxTop cmdlet is a bit of hard nut to crack.
I did some attempts, see here 

Using the definition in Interpreting esxtop Statistics (section 4.2.1) for %USD, I came up with something like this.
Note that the loop sleeps for 5 seconds, because that is the esxtop interval.

Can you check if this indeed returns 100% for %USD for the disk involved?

$device = 'mpx.vmhba32:C0:T0:L0'

while($true){
  $result = Get-EsxTop -CounterName SCSIDevice | where{$_.DeviceName -eq $device}
  $actv = $result | Select-Object -ExpandProperty NumOfActiveCommands
  $qlen = $result | Select-Object -ExpandProperty QueueDepth
  $usd = $actv/$qlen
  Write-Host "$(Get-Date -Format hh:mm:ss.ffff ) %USD = $usd"
  Start-Sleep -Seconds 5
}