Is it possible to move mongodb indexes to another file system?
To be clear, this feature is only available when using the WiredTiger storage engine.

To use, invoke mongod like so:

mongod --dbpath /tmp --storageEngine wiredTiger --wiredTigerDirectoryForIndexes


In your dbpath, you will see a directory called indexes where indexes will be stored.

Also update yaml as

storage:
  engine: wiredTiger
wiredTiger:
  engineConfig:
  directoryForIndexes: true


Kindly note to try this in test environment once.
Please ask if any further help is required.

Source: https://jira.mongodb.org/browse/SERVER-965