Try to make precise what this non-external thing is or provide evidence for its existence. Just come up with one sentence that says something about it that is neither trivial nor so fuzzy that it could mean anything and nothing. -- But be warned, you will have to pass quite a high bar with respect to non-triviality and non-fuzziness to convince me.

If there were a nice answer to this, the "hard problem" would be a lot less hard. But I'll give it a shot.

Along the lines of Descartes, assuming maximal paranoia about everything, it's the thing whose existence you can doubt the least.

That is, let's follow Descartes's line of thinking about how the external world and even all the ideas and concepts in your mind could, in principle, all be an elaborate lie. Then the only thing you absolutely cannot doubt is the existence of the experience of your own thinking. That experience is qualia.

Checking this definition against your criteria:

- Could it mean anything or nothing? It may be ambiguous in that there might be multiple things whose existence you can doubt equally little, but I'd say they're all probably in the ballpark.

- Trivial or fuzzy? Maybe? Admittedly I invoke the somewhat fuzzy concept of "doubt", which I haven't defined properly, and probably can't. But hey, all reasoning builds on some axioms :)

By the way, are you sure your challenge places the burden of proof in the right place? I feel like your position of "our brains are fooling us" is quite a bit stronger and "less default" than my fairly agnostic position. I feel your position at least owes an explanation of why our brains might be fooling us in such and oddly specific and seemingly unnecessary way.

Can you think of an example where we were not only unable to understand something but were even unable to describe the thing we could not understand?

Vitalism (https://en.wikipedia.org/wiki/Vitalism), and the progression from Newtonian physics, are common examples given for this.

Whether we have been unable to describe the things we could not yet understand, I suppose it depends on what counts as a description. We have always been able to point at the objects and phenomena that didn't fit our theories, sure. Qualia on a subjective level might be fundamentally more ineffable for now, but if we insist on immediate objectivity (and I don't think we necessarily should), we can point at qualia too at least on the behavioural level of "we don't know how or why human brains produce this peculiar behaviour of contemplating qualia".

Just like physicists needed to ask the right questions to improve upon Newtonian physics, I think finding correlations between brain activity and (reported) qualia will help us find the right questions, probably at first along the lines of "what's so special about this specific brain region / process". I doubt we'll manage to answer the behavioural question in satisfactory detail without getting at least some new ideas about how to approach the subjective question.

Does that mean that you would be willing to attribute full consciousness to a CPU with some RAM if it ran some software creating the same patterns in memory as they occur in our brains?

The closer the simulation's implementation is to a one-to-one to physical brain processes, the more likely I would think it is to have qualia. If a sufficiently one-to-one implementation of whatever mechanism turns out to be responsible for qualia is possible with CPU and RAM as they exist today, then it'd follow that qualia (and the behaviour of talking about them) are a "by-product" of certain kinds of information processing, rather than some funky physics shenanigans. I'm entirely open to that possibility.

And yes, my sciencey brain also pattern-matches what I just said with "actually having qualia is unnecessary in that statement!", and I'd be inclined to agree with that if qualia weren't so seemingly unnecessary and elaborate for a mere illusion, and if I didn't actually have qualia :)