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:
<xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>AutomountDisksWithoutUserLogin</key>
<true/>
</dict>
</plist>
Thanks to The Tolis Group and Apple Discussions for providing the solution.