Hello,
I can't make hdparm works on my NSA which runs Debian Stretch on kirkwood u-boot and kernel, so i shift to hd-idle.
If anyone is interested, this is hd-idle 1.05 compiled for Debian Strech armel architecture:
https://sourceforge.net/projects/davidegironi/files/deb-packages/hd-idle_1.05_armel.deb/download
Full instructions of my experience on running openmediavault on the NSA310 is here:
http://davidegironi.blogspot.com/2018/12/debian-and-openmediavault-on-zyxtel.html
Hope this helps!