Can arbiterOnly replica in MongoDB become SECONDARY and what it means?
Some time ago my one of my data replicas which was secondary at a time crushed due to hard drive failure. After I fixed that problem and restarted secondary it went into “Recovering” state. But my arbiter is now “Secondary”


A MongoDB arbiter cannot automatically become a secondary or a primary node, as it does not have a copy of the data set.

If you try to manually reconfigure the arbiter as a regular node via rs.reconfig() you should get an exception similar to:

{
    "errmsg" : "exception: arbiterOnly may not change for members",
    "code" : 13510,
    "ok" : 0
}



  Furthermore, the data directory for this Arbiter shows bunch of files of total size >10GB. They indeed look to me like data files. Are they? What is going to happen to these files when Recovery completes?


Assuming this node is definitely an arbiter,
I would expect those files are unused (check the timestamps?) and are either:


a local database created if this node was incorrectly initialized with an oplog
unused copy of data directory if this node was copied from another secondary or used as a standalone before


You can always log into the arbiter mongod directly to see what data it appears to have.