Spin down of Icy Box Raid Enclosure not working

I am running Ubuntu server 18.04 and I am using an ICY BOX IB-RD2253-U31 enclosure with two WD Black disks (no NAS disks) in RAID 1 mode (mirror). I want to put the two disks in sleep mode / spin them down with hdparm (lowest power consumption sleep mode) with sudo hdparm -Y /dev/sdb but the disks seem to run like nothing happened. I also tried sudo udisksctl power-off -b /dev/sdb: This causes the OS to eject the drives, but the disks are still running (I can hear them running). Is this the fault of the enclosure? And is there any way to spin down the disks completely until needed again?