I have a volume mounted. If I switch to another user, this volume gets dismounted. The problem is that I want this volume to be available for every user (but putting it in their login items is not the solution I'm looking for), so: how can this behavior be prevented? (I'm on Mountain Lion)
You can stop Mac OS X unmounting user disks on log-out. This behaviour is controlled with a default (preference).
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
You may also find this Super User question useful, Automount external hard drive on Mac.