Many Western Digital hard disks do not include the ability to standby the drive when inactive for a timeout period.
This daemon is similar to hd-idle. It polls '/proc/diskstats' to determine whether an hard disk should be spun down. See: http://hd-idle.sourceforge.net
Unfortunately, hd-idle appears not to be maintained and is not prepackaged for many Linux distros. It is also difficult to determine whether hd-idle is actually working. This bash script daemon is intended as a simple, understandable, replacement.