Can I make my external HD sleep automatically? I have an external WD USB hard disk attached to my MBP which I use for backups etc. I've noticed that even when it's completely inactive, but the drive is mounted, that the disk is spinning constantly and the unit never goes to sleep.

In "Energy Saver" options I have "Put hard disks to sleep when possible" but since I have an SSD I guess it's not relevant. I had hoped that this option would apply to the external USB drive.

This is on an old MBP 2009 model. Is this a known issue or perhaps a fault of the drive housing?
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.