This seems to be the answer to my problem...but can someone explain it further???
More or less it should mean that the contacts AFTER REMOVING the cardboard are NOT making contact properly.
The suggested solution is to press slightly the board with a finger (obviously through some insulating material or with a rubber glove/finger protection) in the area above the contacts to make them "touch" and re-issue theF3 2>Ucommand.
If you have VERY steady hand you can very carefully untighten a bit the surrounding screws and re-tighten them.
A good idea would be, before trying this, and with no power connected, to disassemble again the board and to thoroughfully clean the contacts with a specific contact cleaner on both board and drive and use a thinner cardboard/plastic insulator. (if using a plastic card be VERY careful about statics)
Now its's clearer...thanks, I'll do as you said.