In the Resync a Member of a Replica Set Tutorial we suggest to "Delete all data and sub-directories from the member’s data directory. By removing the data dbPath, MongoDB will perform a complete resync. Consider making a backup first."

Perhaps it's useful to add a suggestion to specifically backup diagnostic.data specially in case there is an issue being investigated by support. diagnostic.data is much smaller (~ 100MB) and therefore is easier to backup compared to the whole dbpath.

This directory contains historic performance metrics that may be invaluable for investigating recent events, and this information is not replaced by a resync/initial sync the way that the data is