Posted by: NewRadical12
It might be relevant to mentioned that I transferred hard drives when I got a new Xbox.That's actually incredibly relevant. Here's what you do.
Log into the account that purchased the maps. NOT the account you're using, but the account that spent the microsoft points to play the maps in the first place (I'm assuming you aren't playing on the account that first bought the maps, whether you just switched or the account was a friend's/family member's). Then click here and follow the directions. You shouldn't have a problem after that.
If for some reason that DOESN'T work, then perhaps you're over the custom content limit. Delete a bunch of saved maps, films, and gametypes (screenshots are in their own category and don't contribute to this) and it should work. In Halo 3 you're limited to 151 custom content items. 51 are reserved for screenshots, and 100 for maps, films, and gametypes. Unfortunately, DLC maps count towards that limit. so if you have Cold Storage and 3 of the 4 mappacks, that's 10 of your 100 slots taken. So if you have too much other content, the DLC won't load.