OS X Yosemite attempts to eject external drives after sleep
I used to have the same problem, but since installed an app called "Mountain". It allows me to customize which volumes to eject and also re-connect for me on wake. It is not a free app, but it's worth the $5.99 (as of the time I write this).

http://appgineers.de/mountain/

Note, I don't work for them nor am I connected to them in any way. I just like the app and have had great results with it doing exactly what you're doing and more. (I move my macbookpro from 3 locations, where I have different external volume configurations.)