Halo 3 marathon man achievement?

Question

i need help. i got all 7 terminals (accessed)

even the extra one (cortana terminal.) yet i didnt get the achievement.

*i didnt press a to scroll down the terminals, just LOCATED them and ACCESSED them.

Answer 1

its a myth impossible

Answer 2

make sure u sit in the screen till it turns red wait till it takes u too the new screen and then u can back out and u dont need the secret one teminal i dont do anything