Fastest way to sync (or keep import) 3.5TB data from hadoop to sharded mongodb cluster
Why would you want to copy TB's of data from one environment to another system. There are rich projects available such as Apache Drill (to name few) which can seamlessly access Mongo for querying requirements. Have you explored that option?

When you say you have TB's of data in HDFS, what is the format of the data because this is a critical question to be aware of