Mac hangs on startup with arduino uno plugged in


I am running MAC OSX: Yosemite 10.10


Arduino UNO R3

I can connect via serial port fine but when I restart the computer hangs, I have to unplug the board to restart properly. I am using the setup for an installation so ideally need to leave the system all plugged in..

I see this post on the same subject, which came to no conclusions:

Has anyone an idea what the problem might be?

Many thanks