If your harddrives are idle a lot it is good practice to let them enter standby mode. both to save some energy but also to protect the harddrives of one if its most dangerous enemies. Heat!
To set the harddrive idle time you need to have the command hdparm.
To set the spindown to 20 min use
sbin/hdparm -S 240 /dev/hdb /dev/hdb: setting standby to 240 (20 minutes)
To check if an HD is running or is in standby mode you can to
> /sbin/hdparm -C /dev/hdb
drive state is: active/idle
or if the harddrive was in standby mode the response whould be shown as the following
drive state is: standby