So weird. I was opening a bunch of images and using the add to Photos context menu...after awhile Preview just stopped working. FORCE QUIT wouldn't work. Activity monitor did not show Preview running to force quit it, but it still says in the Dock Preview not responding. So I try to log out, Finder doesn't respond. Tried to Option force quit Preview and Finder no response. I then had to force the iMac to shut down. Then it wouldn't boot. lol So I had to hold the power button down for 15 or more seconds and tried again. This time the iMac booted up. That was a nasty crash. Anyone have had this experience with Preview knocking out the Finder and Mac.