I have a shared NFS datastore mapped to all my hosts. So I found it strange that two hosts in a cluster of 5 were not seeing it. When I would try to map it, I would get the following error:
Unable to create datastore: The specified key, name, or identifier already exists.
This worked for me:
It seems I had an old mapping in place that wasn't showing up in vCenter until I refreshed the storage view to show the 'inactive' mount:
[If the refresh doesn't work, you may need to SSH to the host and run the ESXCLI commands listed toward the bottom of the post]
I found it interesting in that running "vicfg-nas <a bunch of connection options here> --vihost myesxserver -l" did not list this inactive path either. It might have shown up if I connected directly to the ESXi host w/ the vSphere client, but I did not check.
ESXi 5, you'll need to use the esxcli commands like so:
esxcli storage nfs list
esxcli storage nfs remove -v nameOfDatastoreVolume