One often needs to drop an old MongoDB store when replacing it. Ex:
store answer into 'mongodb://localhost/agile_data.hourly_from_reply_probs' using MongoStorage();
Before doing that you would likely want to run a mongo command from bash from grunt, to drop it:
sh mongo --eval 'db.hourly_from_reply_probs.drop();'
However, in this case grunt acts as though the command never returns. Crap!