@Flavio
That's the very same thing as my A600: I can choose to boot in 2.1 or 3.1 (3.9 on an A600 is an overkill: it will run over and kill my 600).
On HDToolBox, click on your preferred boot partition and modify the priority. Leave the preferred as +1 and press enter. Save the settings and reboot.
Next time the system will boot with it. If you want to run the other, reset the machine while pushing the two mouse buttons and select the boot device. Nothing more.
