Codex not popping open

According to the codex entry it was an 'alpha relay' powerful enough to open 16 other relays at once....

In regards to the 'Scholar' achievement, many people overlook the question on The Protheans at the beginning of the game where you're in the Communications Room with Anderson and Nihlus. If you don't ask about them there you won't get another chance later.
The Codex entry I kept missing was the one about The Krogan Wars when you visit the Krogan statue at The Citadel, it popped for me after I finally took notice of it. The Codex entries are carried over into subsequent playthroughs so if it's the one at the
beginning, it should pop for you right afterwards...

yes, at the end of it it will show you where the rest are thats when ya recieve your assassins tattoo

1) Check your gamedisc. Around the center hole there is a ring which reads. Xbox 360 printed in the disc which the laser reads the codex from which tells the system that it is a Xbox 360 gamedisc and when read properly the laser will then begin reading the
gamedisc from the outside in (games) as opposed to from the inside out. (CD/DVD)

If that ring on the game is scratched, or dirty and the laser cant read the codex correctly and The sytem will think that their is a DVD in the drive. So clean the disc carefully around the center codex label with a microfiber cloth (Eye glass cloth or simular)
and be extra careful not to scratch the codex ring then try it.

2) If after carefully cleaning the gamedisc and still no joy. Send in the console for repair as the laser is likely far too weak to even read the codex from the gamedisc.

Check the formats of all the songs. It could be that the codex is not supported by the xbox. also there is an ipod codex you can download for the 360 on the dashboard. This can help solve your problem. When in doubt convert all music files to mp3 or
mp4