I purchased a used 2605dn formatter board off the "bay" and although it had an earlier firmware it too, was met by the Error 79. I noticed the large square formatter chip runs hot - very hot. So hot try holding a finger to it for more than a few seconds, so I got to thinking, what if this is a chip overheat issue. So I installed (hap hazardly) a large passive heat sink intended for a pc motherboard chipset, a "chipset cooler" applied some silver heat sink goop, and tie wrapped it to the top (you can't drill holes in this board to mount it).
So far after some 5 days of on-off operation (with the back off), its printed as expected, it would be interesting to note if anyone else may try this to see if they have success, or if this is just happenstance. It's interesting how warm the heatsink gets.
If this proves successful I will look for a lower profile heatsink, and try to come up with a way of securely mounting it to the formatter board, without having to cut a hole in the steel cover plate. I'm not sure if there is voltage potential on this chip (meaning you may not want to strap down any heatsink with any metal strips anchored the formatter board cage), in this case the Nylon tie wrap prevents any grounding to the formatter board main chip because nylon isn't conductive. Perhaps an HP engineer can provide some feedback.
I wonder if similar issues like this with HP P2015's and HP Color Laser 3600 series printers which have similar malfunctions have similar resolutions?