I have what seems to be the exact same issue with my WD Passport drive connected to Odroid XU4. If I reboot the system the drive spins down and enters standby mode (led slow flashing on hard drive and no whirring disc) but if I play music and then stop music so nothing is playing the hard drive never sleeps again.
Like your issue, if I have rebooted the drive will go idle but then later spin up for no apparent reason and never go back to idle – this is even if I haven’t asked the sever to do anything and I do not have any auto music scanning active.
Sorry i can’t help your issue, but if there’s a solution to fix I’d be grateful to know. My drive has been spinning constantly now for weeks and am concerned it will wear out faster in this state.