Probier das, es sendet auch eine Notification wenn ein Fehler auftritt

set startupDisk to boot volume of (system info)
set allDisks to list folder "/Volumes"
repeat with aDisk in allDisks
  try
    if contents of aDisk is not startupDisk then do shell script "/usr/sbin/diskutil unmount " & quoted form of ("/Volumes/" & aDisk)
  on error
    display notification "The disk " & aDisk & " couldn't be unmounted" with title "Unmount Error"
  end try
end repeat