I have to report that is not strictly necessary, as somewhere reported, to have a powered usb hub to fix the problem.
I bought a seven ports hub that can operate with or without external power ( http://www.digicom.it/digisit/prodotti.nsf/ITProdottiIDX/hubusb207P?OpenDocument ) and the previous clicking in integer mode seems to be fixed without ext power also.
Mac mini (2006) with core 2 duo 1.66 GHz, 2GB ram -> usb hub -> arcam rdac.
With kind regards.
Stefano