how to migrate your mongodb into a bigger volume

First stop your mongodb

sudo service mongod stop

Assuming that your existing mongo installation is on /var/lib/mongodb/

sudo mv /var/lib/mongodb/ /your/preferred/destination/for/mongo/

set the rights correctly

sudo chmod -R mongodb.mongodb /your/preferred/destination/for/mongo/

and update the mongodb configuration file

# dbpath=/var/lib/mongodb 

dbpath= /your/preferred/destination/for/mongo/