Can I manually put an external hard disk to sleep? Is it possible to manually put an external hard disk to sleep? Right now, I've already got "Put the hard disks to sleep when possible" checkmarked in the Energy Saver preference pane, and so they should sleep after 10 minutes of inactivity, but I'm wondering if it's also possible to do this manually whenever I want.

I just bought an external hard drive and it's already spinning constantly, compared to my current external which is completely quiet, so I'm wondering if I can put it to sleep when I need to to stop it from spinning all the time.
Not an exact answer to your question, but when using Terminal.app there is an option to even further reduce hard disk sleep time from 10 minutes to 1 minute, with this command:

$ sudo pmset -a disksleep 1