What about the other way, where the external flash IS installed (and the camera is successfully communicating TTL focal length data to it) yet the camera still doesn’t believe there is an external flash installed? Sometimes twisting the external flash a little causes the camera to change its mind and then it works, but then it usually forgets by the next exposure.

Think this mechanism could be causing that behavior too?

Fantastic guide - thanks so much!!! Mine was a little bit different than yours though:

In step 3, there’s another screw parallel to the hard drive tray screw that holds the top case to the bottom that also needs to come out

In step 13, you also need to unplug the blue and white microphone cable (near the top center screw), not in Step 16 (short wire)

Random question - what do you end up doing with the things you tear down after? Do they all go back together again (and end up in full working condition)?