I’ve gotten all of the terminals in solo campaign on my own home console. When I click ‘extras’ then ‘terminals’ from the main menu, I can see all of the terminals and view all of the videos. Does anyone have any suggestions of what I can do to get the achievement to pop?
Have you tried replaying a level and look at one terminal again?
Maybe the system then finds out, that you already have all terminals and will pop it.
If this does not work:
The other way i can think of is to delete all savegames of Halo CEA from your Harddisc, or where you save them, and then replay ALL levels and watch ALL terminals again. Then you probably will be able to unlock it.