I have the same problem, with my brand new ipad air and mi iphone 4s.
What is happening Apple? I can't synch anymore.
When I go to file > devices, everything is greyed out.
Restarting Mac and Phone does not work. They showed up at first, but as soon as you go to Library, you can say goodbye to your devices.
I run on 10.9.5 / latest itunes. And I had that with the previous version of iTunes too.
Oh, and every time I plug in my devices ( or almost every time), I have to "trust" my computer again. ( I connect them to 2 computers, a mac and a pc at work).
Please check this out ASAP Apple.
iTunes still stays the worst thing you ever made. It is better than in the beginning but still..
Such things shouldn't even happen.
This is a MAJOR CRITICAL BUG.
Thanks for acting in consequence.