We have a 3-node replicaset where I'm trying to replace a node with a new server. To do this I'd like to add the new server to the replicaset and remove an old one eventually.

I'm trying to add a member to this replicaset, but I see a ton of errors in the syncing replica member's log.

"NotWritablePrimary: Not-primary error while processing 'find' operation  on 'database_production' database via fire-and-forget command execution."

On the primary server, where the replica is syncing from I can see a ton of:

"Dropping unhealthy pooled connection","attr":{"hostAndPort":"main-mongo-ams1:27017"}}

The machines can connect to each other and it seems some data is being synced, albeit very slowly.

The primary runs 4.4.5 and the secondary 4.4.5, but I also tried 4.4.9 with the same results. 

Any idea what's going on?