Hello
many thanks! Turns out I am an id...
It is even easier than that. The NFS full-mount description is already the MOID. The reference to ha-datastoresystem is working just fine in this combination.
e.g.
<RemoveDatastore xmlns="urn:internalvim25">
<_this xsi:type="HostDatastoreSystem" type="HostDatastoreSystem" serverGuid="">ha-datastoresystem</_this>
<datastore xsi:type="Datastore" type="Datastore">192.168.110.110:/mnt/NFS_HDD</datastore>
</RemoveDatastore>
Again, many thanks - Only now I started using the mob view on host and than it became obvious 