$diskdrive = gwmi win32_diskdrive | ?{$_.interfacetype -eq "USB"}
$letters = $diskdrive | %{gwmi -Query "ASSOCIATORS OF {Win32_DiskDrive.DeviceID=`"$($_.DeviceID.replace('','\'))`"} WHERE AssocClass = Win32_DiskDriveToDiskPartition"} |  %{gwmi -Query "ASSOCIATORS OF {Win32_DiskPartition.DeviceID=`"$($_.DeviceID)`"} WHERE AssocClass = Win32_LogicalDiskToPartition"} | %{$_. deviceid}

$drive = gwmi win32_volume | ? {$letters -contains ($_.name -replace "\")}
$drive.DriveLetter