Prompting personal warning before putting my Macbook Pro to sleep
You could have the drive automatically ejected when you sleep the mac instead?

Quick search found Jettison on the Mac App Store
http://itunes.apple.com/us/app/jettison/id447430809?mt=12

There is also an app you can use to run Apple script on sleep/wake:
http://www.lagentesoft.com/scenario/index.html