External drives not auto scanning (NAV 12)

I just installed Norton Antivirus 12 for Mac on my new MacBook with Retina Display. I'm running OSX Lion (10.7.4). Although I've set Automatic Protection to Enable External Disk Scanning, every thumb drive and CD I've inserted since installing has not been scanned unless I initiate a manual scan from the Status screen. Is there an incompatibility on automatic scans in Lion or with the MacBook with Retina Display?

Norton AntiVirus 12 does not auto-scan disks when they are mounted on the Desktop. We have replaced it with a more secure scan on open technology for files on external disks. The short version is, this feature is no longer needed because we can now more securely find viruses and other malware on your external disks when you try to use the files, instead of performing a long and lengthy scan every time you plugged in that external 1TB hard drive!