I am an experienced cook, more than 50 years in the kitchen. I recently attempted to make a baked potato (basic Idaho baking potato) to go with my meatloaf. I gave it 1 hour and 15 minutes, which has always been plenty. It was hard as a rock. I gave it another half hour, and it had softened slightly at the ends. I have never had to bake a potato longer than 1 1/4 hours. The oven was set at 350°F and is generally accurate. Why would a potato not cook completely after 1 hour and 45 minutes?
All kinds of potatos soften by the time they heat to around 200F. Oiling the skip helps with the heating (it heats fast and "envelopes" the potato with heat). But even without oil, what you describe doesn't make sense.
Is it possible that the over didn't really heat to 350F? Could it be ill-calibrated, unclean (layers of dry fat and whatsnot that gather with the years could make the oven walls more... insulating. Sorry for the bad english), or maybe the heat escapes in another way? Some times the oven's thermometer is totally fine, but located in an area that's hot, while other areas in the oven don't get the heat, from the above reasons or other.
Here's how I suggest you check this: Get a cooking thermometer, the one you stick in meat. Stick it in the potato and follow its heat. Not the heat of the oven but the heat of the food. If it doesn't get to 200F within the hour, your oven is probably not 350F.
don't understand why your tatter wont bake.. I assume you poked holes in it and oiled the skin up a bit.. when I make my peapa big daddy meatloaf.. I used a pan large enough to house my five pound loaf and room on both sides for 6 large bakers. (three on each side) when the meat loaf is done, so are the potatoes.. also throw carrots in there too..