Mac hangs on startup with arduino uno plugged in

Hi,

I am running MAC OSX: Yosemite 10.10

with

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: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiRsO3C8f7LAhXL6RQKHQFEDlwQFggdMAA&url=http%3A%2F%2Fforum.arduino.cc%2Findex.php%3Ftopic%3D99095.0&usg=AFQjCNEHMsd4htNbIv-ACeN92J8_jRjigQ&sig2=i8XEmMYAUoHy_ST6wjWG3A

Has anyone an idea what the problem might be?

Many thanks

Steve