To Backup mongo DB : 
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Meteor db host is at 127.0.0.1 with a port of 3001. Exit the mongo shell and use mongodump from your terminal.

mongodump -h 127.0.0.1 --port 3001 -d meteor
Dumps will be located under the dumps folder in the folder you executed the above command.

You can import your db back to meteor with

mongorestore -h 127.0.0.1 --port 3001 -d meteor dump/meteor