Hmm...here's a weird thing...
First thing to note: I'm using neek2o. I have more than one nand, but I use one as "main nand". Also, before I figured out you need to install cMIOS to the real NAND, I installed one (rev12) to my neek main nand (it's still there because I'm lazy like that). Since then, I update the cMIOS revisions on my real nand and all was fine (oh, I'm using postloader, in case it's important).
Now...I just installed r21, and after a few tests (Kirby air ride & Luigi's mansion still lock up) my neek2o configuration got a little messed up and used another nand instead of my main nand. By itself, it's not so bad (just switch back to my main nand). However...when I was using that other nand, all my GC games showed that "a disc error has occurred" message.
That kind of puzzles me. I was under the impression that even on neek2o, you need to install (& update) DML on the real nand. So how come it gives an error if I use a NAND that probably still has the same old regular MIOS?
So...really which MIOS do I need to update in order to truly test r21? (just the one on real nand, just the one on my emulated nand, or both?)