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?