How did you do the overlay? USB PenDrive? make sure it's firm and you are not bumping into it. If you used a spinning HD you will need a powered USB hub because the host USB plug on the Tian will probably struggle with a spinning HD.
Make sure you are powering the board with at least 500mA, 1A to be on the safe side.
Try removing the external memory and rebooting and test the bridge w/o the overlay. Then put your external memory back and reboot and the overlay should remount. Then try birdge to see if that works. Just after reboot. If it doesn't work with the overlay try /usr/bin/reset-mcu and see if the bridge starts working after the microcontroller restarts.
Finally, try reverting back to whatever it came with.