A couple of weeks ago I noticed my HP printer was no longer printing two sided pages. Strictly speaking this was with Linux but it could happen with the Mac as well since they use the same printer driver software… I figured it was an application problem with Thunderbird and Firefox since all the other applications (and computers on my LAN) worked fine. It never occurred to me it was a driver problem.
Thanks to some prodding by Adam T. at one of the computer clubs I attend, I thought I’d give it a try since there was nothing else left to try. I found some information at https://bugs.launchpad.net/ubuntu/+source/ghostscript/+bug/885118 which seemed relevant. On this page they suggested accessing the CUPS driver directly by accessing the HTML page at http://localhost:631/. This opens the page on the local computer rather than heading out to the internet…
From there I found the defaults were altered by an update. Sure enough CUPS thought it was a single sided printer. I told it to set the default to print on both sides of the page. It whined and said the duplexer was not enabled. The next thing I did was to enable the duplex unit. At that point it accepted the changes.
So far I was only able to print a single six page document from Firefox, but it looks like this solved the problem.