Lumia 720 repeatedly restarts and stuck at gear screen and making no progress while updating from 8.1 to 10. What to do?
Similar behavior happened to me when I had a micro-SD card in the phone that was not properly formatted. I had used it previously to flash an iot-core image and the phone was only seeing a few kb on the card. After I reformatted (FAT) and auto-resized the card with the SDFormatter tool, this problem went away.