I was having this same issue and I found the following post:
http://adminotes.blogspot.com/2012/11/tips-vmware-cluster-remove-datastore.html
I had been trying to remove the datastore via "Inventory->Datastores and Datastore clusters" (popped error for both hosts to which the datastore is attached) and also through "Inventory->Hosts and Clusters->Configuraiton->Storage" for the individual hosts. I tried the unmounting from the "summary" tab per the article above, and it worked. The interesting part was that I tried using the summary tab for the second host, and it failed with the error again. I then went to the Configuration->Storage page and tried removing it from there, and it worked. Weird
(Sorry for not logging a support incident, vmware)