"All is not lost" vs "Not all is lost"
"All is not lost" does not suggest that nothing is lost. It is said to counter the statement or belief that everything is lost, despite appearances to the contrary. It expresses exactly the meaning you say you want to convey, i.e., "at least one thing is not lost."
"Not all is lost" is simply another, more emphatic way of saying the other. It emphasizes the negative and is a stronger counter to any contrary assertions that have been made.
That's the meat of the matter; now to the seasoning: No. 1 can also seem more optimistic, and may be used to suggest that victory is in fact possible. Similarly, No. 2 may sometimes be used in a bitter, pessimistic way to point up how little is left. It may mean that everything of importance has been lost, but there may still exist a consolation prize, meager though it may be. "Our house burned to the ground, but at least the dog house remained untouched," one might say ruefully. "You see? Not all is lost."
I'm with Robusto on this; "All is not lost" is the idiom used specifically to counter "All is lost". "All is lost" -> The situation is hopeless, we're doomed, there is nothing that can save us. "All is not lost" -> The situation can be salvaged somehow, we can make it through this, there's still hope.
"Not all is lost" (if I ever heard it used) would be the counter to "I've lost everything! It's all gone, I can't find any of it!" "Not all is lost; look, here's your paddleball, and here's your chair."
The usual sentence is "All is not lost", as in "there is still hope".
"Not all is lost" is either:
- a dramatization ("All is lost! Not.") used for instance in journalism for big headline ("Not all is 'Lost' in education; ABC show delivers many teachable moments")
- used in the sense of "Not all" "is lost" (which is what you want to convey here), as in "not all is lost in the translation": there is still "something".