Unhappy [Q] GS3 not recongnised in UL Modus from PC

I am using Android 4.3 (Buildnumber: JSS15J:I9300XXUGML29) and wanted to root it.

If I connect the phone via USB to the PC in normal modus it will be found an recognised (with and without USB debugging switched on) by the PC and the port connected to will be shown by Odin.

As soon as I go into upload mode it will not be seen by the PC (again with and without USB debugging switched on) and Odin does not show anything.

Thanks for any help in advance.

MTJ