How do you separate mounting disc from rear view mirror

[RR711]

When my cracked windshield was replaced the installer removed the rear view mirror with the mounting disc still attached to the mirror arm. I have been trying to separate the disc from the arm to reinstall the mirror and can't figure out how it comes off. Also the disc surface that is to adhere to the windshield has 3 tiny points on the surface. It seems that the 3 little points would prevent the disc from lying flat against the glass surface and not adhere. What am I missing? Stumped!