• The resignature fails.
• In the vpxd.log, you see the error:
  
  [VpxLRO] -- ERROR task-1640 -- host-9 -- vim.host.DatastoreSystem.resignatureUnresolvedVmfsVolume: vim.fault.VmfsAlreadyMounted::
   
• In the vSphere Client event log, you see entries similar to:
  
  fault.VmfsAlreadyMounted.summary

This issue occurs when:

• You attempt to re-signature a snapshot LUN which is already mounted/online. 
• The hosts in the DR site don't have the identical copies of the same LUNs/volumes .   

To resolve this issue, perform one or both of these options:

• Unmount and detach the active/online volume prior to resignaturing the snapshot LUN. 
• Ensure that the hosts on the DR site have only 1 copy of each snapshot LUN and avoid multiple copies/presentations.  

Once a VMFS backed LUN is detected as a snapshot, there are two options for mounting/interacting with them: 

•  By force mounting the snapshot LUN, the LUN is enforced to register itself with the existing VMFS signature or UUID reference. 
•  By resignaturing the snapshot LUN with this option, the volume is resignatured and essentially providing it with a new UUID reference. 

The first option, of force mounting, is to be utilized when the original volume is no longer visible as a datastore. By doing this, the VMs that were part of this datastore remain intact and there would not be any need for re-registering them. 

With the second option, of resignaturing, a new volume is created entirely. This would require the VMs on this datastore to be re-registered once again as the path the *.vmx file now has an updated UUID reference. 

In order to proceed with the resignaturing option, ensure that the volume in question is not already mounted/online on other hosts. It is by design that when a snapshot LUN is mounted to a host, the vCenter Server prevents the re-signature of same LUN.