When you press the key combination to trigger the diagnostic menu (calc off, hold OPTN+EXP+AC/ON) the following popup appears:

=DIAGNOSTIC MODE=
Factory Use Only!

 Press : [EXIT]

The way it says it's for factory use only, coupled with the fact that it tells you to press EXIT and gives no indication on how to open the actual menu (F1, 9) makes me think Casio has some reason why they wouldn't want the end user entering the diagnostic menu. Why would this be? Sure, it clears the main memory, which could be an inconvenience, but as soon as you press F1 it asks you to confirm.

Um, most diagnostic menus are "For Factory use only." It just means there are diagnostic features in there that no user is ever supposed to need. You can ignore the message.

It's just that the way it's worded makes me think Casio would rather users not do that. And I ignore the message anyway; they can't tell me what I can and can't do with my own calculator, lol.

There are some interesting features in them though. Search for the threads on the diagnostic menu for the nspire and you'll see what I mean.

I just looked up the thread on how to access it, and I did it on my 1st-gen CAS, and I don't really see anything in there except for the basics--testing memory, display patterns, keyboard test, SW version; stuff like that.

That's pretty useful stuff, actually.

I don't see how it would be useful for hacking the calculator other than the software version, which you can get without going into the diagnostic menu.