[{"name_lower":"total","name":"Total","mounts":[["mount1","instance1"],["mount2","instance1"],["mount1","instance2"],["mount2","instance2"]]}]

db.collection.aggregate([{$unwind:"$mounts"},{$match:{"mounts":{$ne:"instance2"}}},{$group:{_id:"$_id",name:{$first:"$name"},mounts:{$push:"$mounts"}}}])