I had an error 79 and I replaced the formatter with one I had from another HP26015dn out of use, but that changed nothing : still error 79.
What I did is to take out the USB connection out of the multiple switch and to connect it directly in a switch on the computer
and to restart the computer.
No more 79 error and my HP2605dn work perfectly.
I don't know if this was my mutiple external USB switch that was wrong, or if the simple fact to restart the computer did the job.