here's something that others hopefully find helpful I was having random issues with programming my Arduino and yesterday I got to the bottom of it.
I also own a Suunto watch and have their MovesLink2 software installed on OS X for synchronisation. Turns out that if one connects an Arduino, the Suunto helper app not only occasionally claims the serial port, but also starts to send random garbage to it. Needless to say, this causes all sorts of issues programming the Arduino or using its serial port while it is connected.
The solution is simply to quit the MovesLink2 helper app before connecting the Arduino.
I have also filed a support request with Suunto to fix their buggy software (why they do not check that any connected USB device is actually one of their products is beyond me).
Happy new year!
PS. this has been reported before, so this post merely confirms that Suunto haven't yet fixed this issue, and that I am unable to use the search function correctly