Oren Wolfe wrote in to ask about a video playback problem:
Why is my 2011 Mac mini unable to play HD movies on my HP Z27 (known to be HDCP compliant) display? Why is OS X not compliant with HDCP?
HDCP (High-Bandwidth Digital Content Protection) is the digital-rights management (DRM) protection protocol that’s supposed to keep digital films from being easily copied when they pass in perfectly precise form from a player—originally a DVD, and later computers and other devices—and a display of some kind. This can include an integral monitor in a laptop or all-in-one computer, or an external monitor or HDTV set. HDCP is a kind of “lock” on a video cable; otherwise, this would make piracy much easier.
Read more here:: How to fix HDCP errors with external monitors in OS X