If you’re looking in Yosemite for where the Put hard disks to sleep when possible setting in Energy Saver lives, it’s in /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist, so you can view with the command
defaults read /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist
and you’ll see the value in “Disk Sleep Timer”, with 10 meaning the box is checked and 0 meaning it’s not checked.
You can change that setting using
sudo pmset -a disksleep 10
(or 0 instead of 10, if you’d like) to change it for all power types (UPS, wall outlet, or battery), and then change it specifically for battery with sudo pmset -b disksleep 10
Keep in mind the changes may not show up visibly in Energy Saver until a logout or reboot.