It's a simple functionality: when going to sleep and waking back up, a computer should properly mount and unmount all drives. Yet, I was surprised to learn that MacOS X does not always properly external disk. This was particularly problematic for me, as I would lose my main external disk whenever my computer went to sleep.
Some quick research on the Internet led me to a program called Jettison, by St. Clair Software. It automatically umounts my drives when putting the computer to sleep or shutting it down. The author wisely suggests downloading the trial before purchasing a license. My experiences have been very positive. If you are having problems with drives not properly mounting/unmounting, this is definitively worth exploring.
On a side note, if you purchased Jettison from the App Store, like I did, and want to upgrade, read the instructions on the install screen (see bellow in red).