Since Yosemite upgrade, I always need to "force eject" drives? 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.
You can find out what's using the volume with the command:

sudo lsof +D '/Volumes/volumename'


Note that sudo will prompt for your admin password, and it won't echo as you type. The first column will list the name(s) of the processes using files on the volume. In my personal experience, the culprit is usually "mds" and/or "mds_store". These processes are both involved in indexing the files so that Spotlight can search them quickly.

Sometimes they seem to let loose of disk and allow a normal eject, but sometimes not. Unfortunately, I don't know of a real solution; sometimes you just have to use Force Eject.