I see there have been a number of posts about difficulties with USB / serial connection via Mac OS El Capitan, and some suggestions to do with drivers (that should't be necessary), trying different versions of the IDE, and disabling kext signing in Recovery Mode.
What is curious is that whilst none of the fixes have worked for me, I notice that after booting or restarting the computer clicking on the Apple icon and then 'About this Mac' opens the usual window of info about the computer, but from the moment the IDE tries to upload a programme to a Uno R3, not only does that process never finish, but the 'About this Mac' doesn't work any more. No window opens.
This would suggest that there is something fairly fundamentally wrong with the way the uploader is working, interacting with a basic Mac OS process.
On another Mac, running Mac OS 10.7.5 Lion, there are no problems using the same IDE and board.
(MacBook Pro El Capitan / iMac Lion / a wide variety of IDE versions / Uno R3 Genuino)
Any explanations / ideas welcome.