So after successfully updating my vCenter and ESXi host to version 5.5, I proceeded to add back the NFS mount that had previously been mounted only to be met by the following error message:

The “Create NAS datastore” operation failed for the entity with the following error message. An error occurred during host configuration. Operation failed, diagnostics report: Unable to resolve hostname ‘nas.local”

At this stage, my DNS server was not up (and had not been for a while due to me being forgetful over the last few months) and I was mounting by IP Address so couldnt understand why it kept referring back to the hostname. Now the issue I have is that I have become perhaps too comfortable with the desktop version of the vSphere client so am still learning my way around Web GUI. Things have changed, just ask @nickmarshall9 who mentioned recently in a presentation he is still adapting to the new graphical world of vSphere Client Web style

For some reason, I was unable to see and unmount any dead\stale datastore mounts within the GUI, or the desktop client for that matter. If all else fails, go CLI!

  1. Enable SSH access (Off by default), refer to my previous post on how to do this here
  2. SSH into your box using an ssh client, I used terminal within MacOSX so naturally had to specify the root@ username
  3. run the command esxcfg-nas -l to list the volumes it thinks it has currently mounted (why these results do not filter up to the GUI astounds me)
  4. now to add a temporary entry into the hosts file,   vi /etc/hosts and add <ip address of NFS Server> <hostname>
  5. Save and exit vi
  6. esxcfg-nas -d <stale_NFS_mount>
  7. Delete the temporary entry that you added to the /etc/hosts file in Step 4 using vi
  8. You should now be able to add the NFS volume through the vSphere client
  9. Turn off SSH access

Simple solution, but one for a problem that seems more or less broken in the software stack.