Hi everybody. Thanks for the replies. I'm happy to announce that I did fix the problem. I built a new SD image and on this fresh installation I installed the gphoto2 from te repository with the usual apt-get command. It worked as it was intended to do. Then I realised why it didn't work on the other SD card: before installing from the repository I tried to compile the source code of gphoto2 (so to have the latest release) on my own but the compiling process ended with several errors; only after this unsuccesfull operation I decided to get it from the repository but something must have gone wrong and looks like the two operations made a mess ! Needless to say that I use the command line shell only on Linux and obviously I had already put all the cameras in PTP mode.