I just upgraded to OS X Mountain Lion and now none of my videos will play on Quicktime. That includes videos I just recorded on my iPhone 6 and uploaded to my Mac. What happened during the upgrade to cause this problem and how do I fix it? Most of my videos were recorded with my iPad or iPhone. Thanks.
Specifics of your Mac? Model/year/processor/etc.
QT Player version? Pro?
What exactly happens when you open/play a movie? How do you "open" it? Double-click the file? Open from the QT Player [File] menu?
Can you open/play these movies in VLC <http://www.videolan.org/vlc/index.html>?
{maybe now that the holiday weekend is over, the QT gurus will attend to this 'bump' with benefits - I have no experience with Mountain Lion, much less your specific issue}
Quicktime Won't play videos after upgrade to Mountain Lion
