I think (A dangerous thing to do these days) that CFW uses the higher ARM processor . In turn does not run the code needed for it to run NDS games. So in order to play them you have to go into a official firmware.

 

What? That isn't true at all. I have played ds games on reinand and it hasn't had any problem. Maybe this happened on a very old cfw...

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I think that your flashcart might have an outdated firmware. It could also be that it is an older flashcart that was patched out by nintendo. If this is the case then you should look up the patched TWL_FIRM file and install it. It allows you to use outdated flash carts. If that flashcart is really old then I would recommend this other dsiware hack that fixes that. I cant remember what the name was but if you look around in the homebrew section on gbatemp you should be able to find it. I did this and was able to use my original r4 on my n3ds

 

Did your card work before on this same 3DS?
You should check out this thread:
http://gbatemp.net/threads/twl-slot-1-launcher-first-custom-dsiware-app.414501/unread

 

DS Mode doesn't work in rxtools on New 3DS consoles. Use reiNAND instead.

 

I am using reinand it just said error occured it working ok in dsi