Browse Data store itself and check to make sure no vmk, vmx or other similiar files have not be left behind during storage migration, you will have to migrate it to where VM is at. Also check to see if any orphan files are left which you have to delete manully. Once in a blue moon during storage migration I noticed VMX file and VMDK where on two different luns after migration, one on source datastore and other on distination. But mostly orphan files on datastore that had to be deleted. ALWAYS ensure before you delete orphan file that VM has that file in current datastore. Deletion without confirmation can lead to big prob giving heads up
check for some lock is there. use vmkfstools -L reserve and then try to release it
if you have already checked your Datastore is free, rescan your storage twice then delete your datastore.
My 2 cents
Sam
Did the datastore contain any ISO files? If there are still VM's pointing to the datastore you will not be able to delete it, even though the ISO do not exist anymore. Depending on the number of VM's you have, you may use e.g. RVTools (http://www.robware.net/) to check the VM's settings.
André
Finally was able to get rid of it after a reboot of the host.
Thanks for all the responses!