Check the back of one of the pillars near the Dark Energon pool. There should be a switch to empty the pool. Then, you'll see a passage where a "light" Decepticon can drive through. The disc is through that passage.
If you believe in Jesus Christ as your personal Lord and Savior and are 100% proud of it, put this in your signature.
Glad to know I'm not the only one stuck missing one. One was fairly easy to find so I assume we're missing the same one. I've been through that stage over 10 times and searched practically every last corner of that stage top to bottom and haven't found it. >__<
Whoever said nothing is impossible never tried slamming a revolving door.
@GreenLantern173: If you're only playing story mode, it'll be awhile before you unlock a light character, at least 3 stages if not 4, I can't remember. Alternatively, you can try Arena, I think you unlock a light vehicle class by stage 6, you'll need to fight him though and having a flying character definitely helps in that battle.