Some more info would be helpful.
What type of Mod? What Bios? Are the IGR's turned off in Evox System Settings? Where is you HDDLoader located, and how is the Evox.ini menu set up?
If it's an older BIOS, maybe HDDLoader needs to be signed to be run? It'd have to be a pretty old bios though, but also I think most people are using DVD2Xbox over HDDLoader at this point.
I'd make sure HDDLoader is in F:\Apps\HDDloader\ on your xbox and that your evox.ini specifies AutoAddItem F:\Apps\ in the menu structure (not sure if that's the exact command tho, you'll have to check that.)
But, just make sure that it isn't trying to run it from somewhere that it doesn't exist.