Hopefully the previous responses were correct in interpreting the OP's question to mean "where do you put your emergency fund?" (ER = emergency reserve?).
Todd, your use of MMM catch-phrases like "little green soldiers" and your prior posting history lead me to believe the above interpretation was correct and you probably already know the following, but, in the off chance you were actually asking "where do you put your early retirement fund" (which, in my view, is really another way of saying "your life savings" or "all of your money"), then the answer is definitely not "a 0.0000185% savings account," and the responses you'd get would significantly differ from those appearing in this thread so far.
Alternatively, maybe it's the case that your taxable accounts "only" have ~$20k in them right now (in which case you might view the entire contents of those taxable accounts as your "emergency fund"), but if and when those accounts accumulate enough to exceed what you view as your "emergency fund," then your investment strategy for those dollars will need to reflect the fact that you intend to live off of them (plus the amounts in your tax-deferred accounts) for the rest of your life, post-retirement.