How to Disable USB Auto-mount
OS X has a mechanism, called the Disk Arbitration framework, through which programs can control what happens when disks are inserted or ejected.  It also provides an opportunity to intercept a disk insertion event and prevent the usual automatic mounting behaviour.

To change the behaviour, though, requires a third-party utility, since the Disk Arbitration framework is just a programming interface.  One good utility is Disk Arbitrator, a free (BSD-licensed) tool by Aaron Burghardt.



While Disk Arbitrator is running, you can choose to either veto the mounting of newly inserted disks, mount them read-only, or just let the normal automatic mounting happen.  After Disk Arbitrator exits, the normal mounting behaviour is restored.