Can't use vMotion Storage between two ESX hosts

Hi all,

I'm currently administering two vSphere 4.1 hosts (both with vMotion AND Storage vMotion), which are managed by

a vCenter 4.1 host.

Both hosts are known by the vCenter server, however when a VM from one host is dropped onto another the following

error appears:

This error should not even be appearing, both hosts are capable of Storage vMotion - does anything in particular need to be performed to the datastores so one ESX server's datastore is available to the other?

james