HDD doesn't recognize hdparm -S flag

My WD6400AAKS is attached to a Raspberry by USB. It is contained in an external USB-case with access LED. It works fine on hdparm -y and spins down. If you do an ls on that drive, it spins up. So far so good. Unfortunately it doesn't recognize -S. Tried -S 1, -S 12. hdparm returns correctly with no error, but drive won't spin down, though no access is indicated by LED. Any idea?