I simmered it all night, cooled it immediately in two small containers, and didn't end up needing all of it. May I still divide it into even smaller containers and freeze it safely?
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Six days is past the recommended (refrigerator) storage time. The USDA recommends that you discard it. If it were me, however, I'd bring the stock to a boil for 10 minutes then cool in an ice bath before freezing. There are those who will argue against doing so and I think that's fine tack, too. It's hard to go wrong following USDA recommendations.
Here is what Harold McGee has to say about the situation:
"a fine tack"
Freezing will suspend the stock's current microbial state which is probably fine but technically suspect at this point. Boiling will reset the clock so to speak.