There is quite a lot of confusion around this achievement and I thought I'd share the things I found out:
- The Bungie link that is famously posted everywhere is incomplete
! Do not use it.
- The most complete guide is: http://www.fantasiadomain.com/marathon/marathon.html
- The fantasiadomain guide has one omission: #1 is missing from the map on level 3. It's in the same cluster of terminals as #2 and #4.
- Never dashboard out of the game. Quit to main menu and then quit the game from there, in order to make sure your terminal progress saves. It does save which terminals you've found so if you missed one on level 3, for example, you can just play there and get that one.
- Most important thing
that I haven't seen any other guide for this game mention: you need to read all messages! The achievement is not for simply finding all terminals! In the guide above, you'll see them labeled as U and F. U is the first time you see the terminal, F is if you backtrack to that terminal after completing all the level objectives, and the terminal will then have a different message. I'm 90% certain that this is why most people seem to fail this achievement and why the ratio is so high.
Here's exactly which terminals I read in which levels:
01. Waterloo Park - ALL
02. The Slings & Arrows of Outrageous Fortune - NOT F
03. Charon Doesn't Make Change - ALL
04. What About Bob? - NOT F
05. Come and Take Your Medicine - NOT F
06. We're Everywhere - NOT F
07. Ex Cathedra - ALL
08. Nuke and Pave - ALL
09. Curiouser and Curiouser... - NOT F
10. Eat It, Vid Boi! - ALL (NOT U ON FINAL TERMINAL)
11. The Hard Stuff Rules... - ALL
12. Bob's Big Date - ALL (NOT U ON FINAL TERMINAL)
13. Six Thousands Feet Under - ALL
14. If I Had a Rocket Launcher, I'd Make Somebody Pay - ALL
15. Sorry Don't Make It So - ALL
16. For Carnage, Apply Within - ALL
17. Begging For Mercy Makes Me Angry! - ALL
18. The Big House - NONE
19. This Side Toward Enemy - ALL
20. God Will Sort The Dead... - ALL
21. My Own Private Thermopylae - NOT U
22. Kill Your Television - ALL
23. Where the Twist Flops - ALL
24. Beware of Abandoned Rental Trucks - ALL
25. Requiem For a Cyborg - ALL
26. Fatum Iustum Stultorum - ALL
27. Feel the Noise - ALL
28. All Roads Lead To Sol... - ALL
"ALL" means I read both U and F versions, "NOT F" means that I didn't backtrack to read the terminals again when the level was done and "NOT U" means that I missed reading a terminal before completing the level. As you can see, I didn't think to backtrack on the first 9 levels and I have never done so on this save file.
However, I did play through this game once many years ago when it came out, so I can't guarantee that I didn't read the U/F versions of the terminal back then. To be safe, make sure you read all versions and if your achievement didn't pop, that's probably why.