I've connected an external Seagate Backup Plus Portable 5 TB hard-drive via USB 3.0.
The drive and USB-to-SATA interface it capable of being put into standby by the OS, as Windows 10, Ubuntu and even the FritzOS on my router can automatically and/or manually spin-down the hard-drive.
Unfortunately, when using OMV, this does not happen. I've already set the following values:
Did I miss a setting somewhere that I still need to change?