After installing snow leopard on my 2.8 GHz 8-core Mac Pro, I cannot do duplex printing. I have also installed the 10.6.1 update. I presume snow leopard thinks my printer driver is up to date, since I have not received the HP printer driver update to install. I have even downloaded the HP Printer Drivers for Mac OS X v10.6 update from the Apple Support downloads site, but I get the following "error message" prior to the installation completion: System extension cannot be used The system extension "/System/Library/Extensions/hpPlugInInit.kext" was installed improperly and cannot be used. Please try reinstalling it or contact the product's vendor for an update. There was a mention elsewhere of a check box in "print & fax" in system preferences to enable duplex printing. Unfortunately, I cannot find this check box.