Ok, so last night I went to see this dude, he modded my 360 (Hitachi 79FL) and popped in a copy of CoD: World At War, and it booted up (awesome game BTW), and worked fine... I got a few levels into it. So after that, I stuck in a few different copies of numberous games that are infact backward compatible. However, the backups of the original Xbox titles won't boot (Fable, Mortal Kombat Armageddon, Burnout 3, King of Fighters: Neowave, etc....) By the way, I DO own the originals, it's just they are scratched like crazy (hence the backups), and my 360 refuses those because of the scratches.
Anyways, when I ripped the games, I used DVD2Xbox on my original Xbox, and ripped them as ISO files, FTP'd them to PC and burned them. They play perfectly fine in the original Xbox, but the 360 hates them.
I'm not an expert, yeah, but I'm fairly certain that the problem is the media. You likely have the games burned on single layer dvd+r and for the 360, all games including original xbox 1 backups, need to be on dvd+r dl. I'm pretty sure I read that somewhere while learning to burn 360 games.
I had a slight inclination that's what it was, but I was hoping it was something else... well, at least I know it's nothing major and nothing wrong with the mod... the only biatch of that would be that I gotta get Dual Layer discs, and have to re-burn everything.