My hard disk won't spin down using hdparm -S24 /dev/hdb
 

I tried to put the command in /etc/hdparm.conf but it didn't work. I tried to issue it manually and it didn't work either. I tried to force the disk to enter standby mode and it worked, I used hdparm -y /dev/hdb. I need to set a spin down timeout, how can I make hdparm -S24 work?
Thank you