Although not technically backcountry, I do live pretty much in middle of woods almost in dead center of Maine and caught this guy stealing food from birdfeeder at night. It was not eager to leave when I approached but very chatty (and obese).
We have one of these that lives in our shed (in Rhode Island). Those big eyes staring at me from a dark corner have scared the crap out of me a couple of times.
Flying Squirrels are a major PITA whens it comes to hanging food. Many years ago camping near the old Desolation shelter, I was woken up by an odd noise. It was a flying squirrel gliding over and chewing a hole in my food bag. The bag was waterproof and the gorp inside was in a ziplock but the squirrel drilled right into it like he had radar. Folks uses "rat sacks" to keep them out.
We had a big problem with flying squirrels that had taken up residence in the wall at the head of our bed in our cabin... scratching and rustling around all night long. Had to rip the wall apart to finally evict them:
Every once in a while we get a gray squirrel that all but flies from a nearby pine tree to our bird feeder ... despite all my pruning of that tree to remove such temptation. Most squirrels don't even attempt the leap but the occasional tarzan gets rewarded with an unlimited supply of black oil sunflower seed. When there is such an athletic squirrel we have learned not to worry about the seed that's being monopolized ... they soon get too fat to make the leap.