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Greetings /\/\o\/\/
I found a work-around :
(I almost give te solution in the last post
Setting the Path / system variables from MSH )
first create the Key, then Set it using WMI (so it gets updated ;-).
new-property -Path "HKLM:\System\CurrentControlSet\Control\Session manager\Environment" -Property New -Type string -Value "foo"
$env = get-WMIObject win32_environment -filter "Name = 'New'"
$env.VariableValue = $env.VariableValue + "Val"
gr /\/\o\/\/