What causes the error unable to unmount/delete VMFS Datastore: the resource is in use?
The issue occurs if the datastore being deleted is being actively used/referenced by the VMware ESXi host.
Based on the error, it is clear that the VMFS datastore cannot be removed since ESXi hosts or vSphere are still using storage to write some data.
https://ibmimedia.com/blog/212/tips-to-fix-unable-to-unmount-delete-vmfs-datastore-the-resource-is-in-use
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