Please have a look at this thread. ...https://forums.adobe.com/message/7692856#7692856
Here is the main message..."We’ve identified a shift in OS 10.10 behavior that seems to be tripping us up and are testing a fix to roll into our next release. Unfortunately the only workaround until then is to downgrade to the previous version of Lightroom. Thanks to all of you who provided details to help us track this down."