How to let HDD standby/spin down?

In physical disk section, I already chose the power management options for all data disks, I logged out from the system for > 40mins (I configured the standby time to be 20min) but still feel one of the drive boiling, the other one without any mount point is good (both are same model), is there anyway I can let the drive with mount point sleep/standby?

root@openmediavault:~# hdparm -C /dev/sdb

/dev/sdb:
drive state is: active/idle
root@openmediavault:~# hdparm -C /dev/sdc

/dev/sdc:
drive state is: standby
root@openmediavault:~# hdparm -C /dev/sda

/dev/sda:
drive state is: active/idle

sda is the OS drive, sdb is the mounted drive (ZFS), sdc is unformatted drive

so no way to make it standby?