OK, this one was really annoying. I was able to print successfully using my HP printer but hp-scan/xsane would give me this error: "Error during device I/O." Then it would walk me through installing the proprietary plugin, which I had already installed from
http://www.openprinting.org/download...es/HP/plugins/.
I ran hp-setup, hp-plugin, and hp-check hoping to find something obvious. Then a thread reminded me to check syslog, and I found this:
...
