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

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

(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

Keep in mind the changes may not show up visibly in Energy Saver until a logout or reboot.