Currently the Remove-HardDisk cmdlet comes with the message
"The FlatHardDisk 'xyz.vmdk' is not attached to a VM. The requested operation is only supported for devices attached to VM"
when applied to a zombie VMDK.
While Get-HardDisk has a DatastorePath to access such zombie VMDK, the Remove-HardDisk cmdlet does not accept such a HardDisk object.