Whenever I have a hard drive plugged in the computer (mid-2011 iMac) that I want to eject, I get the message "unable to eject the volume, an application is using it, would you like to force eject?"
I believe this is new, it didn't do it before when I wasn't under Yosemite.
What is the "application" using the volume? (in most cases)? I am sorry if this is too little information for now to troubleshoot it, I am willing to share more info on my current settings - just tell me how.