If you are running PGP Desktop, make sure to update to version 10.2.1 (released 5/7/12) prior to updating to OS X 10.7.4. If you don’t, the PGP Desktop will fail to mount Virtual Disks, and updating after the fact won’t fix the problem. Symantec is developing on a fix, and has posted a workaround here to allow you to access your data: http://www.symantec.com/business/support/index?page=content&id=TECH188695