By default, OS X only mounts external drives (USB, Firewire, Thunderbolt, etc.) when you log in, and unmounts them when you log out or reboot. To change this behavior, enable AutomountDisksWithoutUserLogin
.
sudo defaults write /Library/Preferences/SystemConfiguration/autodiskmount AutomountDisksWithoutUserLogin -bool true
Installing OS X Server also enables this setting.
Alternatively, you can create the /Library/Preferences/SystemConfiguration/autodiskmount.plist
with the following content: