Maybe you are not getting responses because you put an essay into the title. The title should be worded in such a way as to attract people who have the appropriate knowledge and skills you need, and preferably contain or moly a question.
Back to the DVD issue. If you are dealing with DVDs from multiple regions, you are only allowed to change reckons on a DVD player five times before it will no longer change.
Still odd that one Mac will play all of the DVDs without complaint while the other does not.
Take a look in System Preferences/Language and Text/Formats. Is the region the same on both Macs?
Also, it is the new design of the forum that leads to people posting "essays" as the title. Whatever you type into the "ask the community" box becomes the default title of the post.
Region Code 4:
Mexico, Central and South America, and the South Pacific.
Includes Antigua and Barbuda, Argentina, Aruba, Australia, Bahamas, Barbados, Belize, Bolivia, Brazil, British Virgin Islands, Chile, Colombia, Costa Rica, Cuba, Dominica, Dominican Republic, Ecuador, El Salvador, Falkland Islands, French Guiana, Grenada, Guatemala, Guyana, Haiti, Honduras, Jamaica, New Guinea, New Zealand, Nicaragua, Panama, Paraguay, Peru, Suriname, Trinidad and Tobago, Uruguay, Venezuela
Apparently my first response was off in left field. The DVD player has its own code. My recommendation is to change the code when it asks. It is possible that the disks that play correctly are region-free and the ones that don't have a region code. If this does not work, you still have four more changes available; you won't be stuck with the new code.