Most SATA disks support power management with out of the box settings. WD disks are different. They come with power management disabled. Enabling power management requires placing a jumper: http://support.wdc.com/KnowledgeBase/answer.aspx?ID=958 Users may want to disable the idle3 timer: http://support.wdc.com/KnowledgeBase/answer.aspx?ID=5357 idle3-tools can be used with Linux.
With both actions performed hdparm -y will put WD drives in standby mode. Accessing the disk will activate them. I also tried to set the sleep time using hdparm -S <some value>. A WD40EZRX failed to spin down whatever value was given on the command.
linux-lfh1:~ # hdparm -S 10 /dev/sda
/dev/sda:
setting standby to 10 (50 seconds)
linux-lfh1:~ #
Tried a wide range of values and did not succeed.