A VMFS datastore fails to unmount although it has no registered VMs or VM tempaltes

Products

• VMware vCenter Server

Issue/Introduction

• A datastore is to be unmounted.
  
  
• There are no VMs or VM templates registered on the datastore.
  
  
• The unmount operation fails with
  
  "Unmount VMFS volume failed! The resource <datastoreName> is in use. Cannot unmount volume <datastoreName> because the device is busy"  

Environment

VMware vSphere ESXi (all versions)

Resolution

Implement the "Pre-Unmount Checklist" in Detach a LUN device from ESXi hosts