I'm running OS X Yosemite and I've got an Uno r3.
It appears that I get a tty.usbmodem showing up in my /dev list. In fact, I can communicate with the Uno. I can upload sketches, etc.. The problem is that it appears to be rebooting in 5 second intervals on the dot. For example, if I upload the basic blinking sketch, it will reset on the 5th interval and start back over. I notice the 13th pin led blink three times and then the blinking starts over again.
I noticed that when I reboot my mac, the Arduino will operate normally. The blinking will remain constant and it will not reset. When I login to the Mac, it begins its vicious reset cycle. I'm at a complete loss.
I've updated the drivers, uninstalled, reinstalled, rebooted, etc. Like I mentioned before. I've got a 5 second window to do my testing until the Arduino resets. I'm at a total loss. I've searched for hours on the internet and I can't seem to find anything that reflects this issue.
-Phil