Fast user switching: how to prevent volumes from being unmounted?
You can stop Mac OS X unmounting user disks on log-out. This behaviour is controlled with a default (preference).

In this discussion on the Apple Support Community, Király shares the appropriate command to issue:

defaults write /Library/Preferences/SystemConfiguration/autodiskmount AutomountDisksWithoutUserLogin -bool YES


This command needs to be entered into Applications > Utilities > Terminal.app. Aftwards you must restart your Mac for the change to take effect.

Note that the autodiskmount tool has been replaced by automountd; it is possible Mac OS X 10.8 has migrated this default to another file.

You may also find this Super User question useful, Automount external hard drive on Mac.