On several Western Digital drives, hdparm doesn’t play so good. That’s why the hd-idle project was created. But hey, that’s C and not very flexible actually. Have a look at my Python rewrite here. It uses sg3_utils sg_start tool and it’s been successfully tested on SATA (over SCSI layer) devices. I don’t know if it’s going to work on pure SCSI devices (given the fact that they might not come back from spindown automatically, IIRC).
Feel free to deploy it on your home server, it really saves a lot of money on the electricity bill.