Well, two answers, it can be configured via your Mac in System Preferences under Energy Saver (see attached). The second is some external drives have a firmware controlled sleep mechanism. In the instance of WD, I believe they sometimes ship with WD Drive Utilities or WD SmartWare (depending on the type of WD drive, i.e. My Book Essential) which you could use to disable the sleep timer.
That being said, perhaps it is not a case of the drive itself going to sleep. Just exploring all alternatives.