Thanks for the tip. I did a freshmeat search for hdparm since it wasn't included with the standard ARM debian distro... And lo and behold look who the author of hdparm is. Should have known!

It works as advertised. hdparm -y does indeed spin down the drive. There are some other tuning parameters but until I know what they do I'll stick to that one.

Anyway I built hdparm 4.4 and I'll attach it here for anyone who needs it.