Umount vs. disk spindown

[badhat]

I've recently migrated from a G5 dual to an intel mini. The mini has an external firewire drive (Seagate mechanism) used exclusively for overnight backup using SuperDuper in scheduled mode. The external drive is left unmounted during the day. When the backup occurs, SuperDuper mounts the drive successfully, performs its Smart Copy, and then unmounts the drive.

The problem is that whlle the drive is now unmounted, it does not stop spinning. I have to launch Disk Utility, MOUNT the drive, and then UNMOUNT the drive to cause it to spin down. It remains this way until the next overnight backup, which executes but leaves it spinning.

I'd like to eliminate the spinning of the unmounted drive for a few reasons (noise, wear, etc.), and I'm thinking this could be accomplished with a script. But I'd rather understand more about why it won't spin down when unmounted. Any ideas - anyone?