Do you install the CEA maps on to your HD and play it off the reach disk?

Games Other Halo
1

Or do you play it off the CEA disk?

2

You play CEA off of the CEA disc. You can play the CEA-playlist offered in Reach through the CEA disc.

CEA and Reach are separate entities.

3

Take the Halo 3: ODST packaging for example, where it had the ODST campaign on the first disc, and the Halo 3 multiplayer on the second disc. Except this time both the Halo CE campaign and the Halo Reach multiplayer are on the same disc. Unlike the ODST multiplayer disc, where it has all the DLCs for Halo 3 on it, the new Map Pack that comes with Halo CEA is the only one you can play on that disc. Unless you have already downloaded the other two DLCs, in which case you can play those DLCs on the CEA disc too.