I have the latest version of Merlin FW install on a ASUS RT-AC68U router, and have connected a USB 3 drive to run media off, but this drive spins 24/7. How do I set the drive to stop spinning when not being accessed?
Have you checked the box for that option in the gui and rebooted?
If all the above has been done (in order) and the issue continues, it may be the drive itself that regulates if it sleeps or not. Did it work as expected in previous (version number?) firmware?