This is a set of scripts we use to reliably drop sharded databases on a MongoDB cluster. The reason for doing this is because mongos is very unreliable when it comes to cleanly and reliably dropping stuff.

remove_db.js

function removeDB(n) {
  flushConfigs();
  var colls = findChunkedCollections(n);
  
  colls.forEach(function(coll) {
    if(!db.getSisterDB(n).getCollection(coll).exists()) {
      print("collection did not exist for chunk: " + n + "." + coll)
      getPrimaries().forEach(function(p){
        p.getDB(n).createCollection(coll);
      });
    }
  });
  
  print("Dropping: " + n + " on all shards");
  printjson(db.getSisterDB(n).dropDatabase())
  
  getPrimaries().forEach(
    function(m){
      print("Dropping: " + n + " on " + m)
      printjson(m.getDB(n).dropDatabase());
    }
  );
}


function flushConfigs() {
  getActiveRouters().forEach(function(mongos) {
    print("Flushing: " + mongos);
    try {
      mongos.getDB('admin').runCommand({flushRouterConfig:true});
    } catch(e) {
      print("Error flushing...");
      printjson(e);
    }
  });
}


function getActiveRouters() {
  var config = db.getSisterDB('config');
  // only return ones that have been alive in the last hour
  return config.mongos.find({ping:{$gt:new Date(new Date() - 60000)}}).map(function(mongos) {
    try {
      return new Mongo(mongos._id);
    } catch (e) {
      return db.getMongo();
    }
  });
}

function findChunkedCollections(n) {
  var m = {}
  var colls = [];
  var chunks = db.getSisterDB('config').chunks.find({ns: new RegExp("^" + n + "\.")}).forEach(function(c){
    var coll = c.ns.split('.')[1];
    if(!m[coll]) {
      colls.push(coll);
    }
    m[coll] = true
  });
  return colls
}

function findShardMembers(shard, state) {
  var urls=shard.host.split("/")[1].split(',');
  var members=[];
  new Mongo(urls[0]).adminCommand({replSetGetStatus:1}).members.forEach(function(member) {
    if(member.state == state || isNaN(state)) {
      members.push(new Mongo(member.name));
    }
  });
  return members;
}

function findPrimary(shard){
  return findShardMembers(shard, 1)[0]
}

function getPrimaries() {
  return db.getSisterDB("config").shards.find().map(findPrimary)  
}

Usage:

load("remove_db.js");
removeDB("sharded_db_name");