Master Reset won’t work (Edge Explore stuck in loading profiles loop)

Hi - I have one of the first Explore models, currently on firmware 4.0. It’s been great, particularly after my old 800. After dragging and dropping a gpx route file into New Files folder (done hundreds of times), when I unplugged the device from my Mac, I noticed something new. Three little dots moving across the screen while it was “Loading Profile”. I left it for several minutes, no change. Having tried Mr google and Mr YouTube, none of the solutions to fix this loop of death appear to work. Most worryingly of all, it won’t respond to the full master reset (hold lap and start/stop buttons, then power up, then release when the garmin logo disappears. I’ve tried several times, holding the buttons for longer etc and still no joy.