Can I make my external HD sleep automatically?
Mac OS X has no explicit option for users to power down external hard disk drives. This capability is managed by Mac OS X's lower level IOKit.

Regrettably, not all external hard disk drives offer energy saving features. However, Western Digital (WD) claim that all their drive spin down when inactive. 

It is likely something is accessing your external hard disk drive. It only requires a tiny read or write to cause the disk to stay active. Maybe a log file being written to or back up software that never stops.

Does the drive spin down when you unmount it in the Finder? If so, consider scripting the mounting and unmounting of the drive as part of your back up routine.