Thanks a lot.
That's what I managed to do after looking at multiple tutorials using arduino as ISP. AVRDUDE is really tricky and from all the solutions I've found, non functioned exactly.
Here is the avrdude instructions that worked for me:
avrdude -P COM4 -b 9600 -c avrisp -p at90usb162 -U flash:w:UNO-dfu_and_serial_combined.hex
- I had to change the Serial.begin(9600) speed within arduino as ISP sketch
- using avrispmkii triggered an error: not in sync
- using usb instead of com port triggered an error
With this command, avrdude warned me about device signature error. I bypassed it using -F instruction.
The firmware was finally flashed.
BUT, once reconnected by USB, I had to reinstall the UNO driver but the board still refuses to enter dfu mode
At least I've learnt how to flash it