$results = Get-Counter "\PhysicalDisk(インスタンス名)\*" -SampleInterval 1 -MaxSamples 60
#インスタンス名は  (Get-Counter -ListSet PhysicalDisk).PathsWithInstances を実行して確認してください。例、 "\PhysicalDisk(1 d:)\*"

$csamples = $results.CounterSamples

$grouped = $csamples | Group-Object Path

$avg = @{}
foreach ($g in $grouped) {
    $avg[$g.Name] = ($g.Group.CookedValue | Measure-Object -Average).Average
}

foreach ($k in $avg.Keys) {
    switch ($k) {
        {$_ -match "% Idle Time"} {$diskidle = $avg[$k]}
        {$_ -match "Avg. Disk Queue Length"} {$adiskq = $avg[$k]}
        {$_ -match "Avg. Disk sec/Read"} {$adiskspr = $avg[$k]}
        {$_ -match "Avg. Disk sec/Write"} {$adiskspw = $avg[$k]}
        {$_ -match "Read Bytes/sec"} {$rbps = $avg[$k]}
        {$_ -match "Write Bytes/sec"} {$wbps = $avg[$k]}
        {$_ -match "Disk Reads/sec"} {$drps = $avg[$k]}
        {$_ -match "Disk Writes/sec"} {$dwps = $avg[$k]}
    }
}

$outstr = "% Disk Time : {0:N2} %" -f (100 - $diskidle)
$outstr += "`nAvg. Disk Queue Length : {0:N2}" -f $adiskq
$outstr += "`nAvg. Disk sec/Read : {0:N2} ms" -f ($adiskspr * 1000)
$outstr += "`nAvg. Disk sec/Write : {0:N2} ms" -f ($adiskspw * 1000)
$outstr += "`nThroughput : {0:N2} MiB/s" -f (($rbps + $wbps) / 1024 / 1024)
$outstr += "`nIOPS : {0:N2}" -f ($drps + $dwps)

$outstr