Ok, found "Refresh Devices" in the Device dropdown menu. But how can I run "Refresh Devices" automatically after a new mapping? I use an external program (Rundll32 Shell32.dll,SHHelpShortcuts_RunDLL Connect) to map a drive. I added as User Command "Map network drive..." and combined it with hotkey F11. I haven't found an internal command "Refresh devices". But even if this would exist, I have no idea how to combine an external with an internal command.
Anyway MC should do the "Refresh Devices" itself without any interactive or user defined action.