Storage Area Networks - both FC and iSCSI - do not automatically show LUNs to all hosts that connect to them. This also called masking and prevents accidental access of LUN by host that is not authorized to access that LUN. You have to configure the storage system granting access to the LUN - once you do this for the other hosts it will show up in the list and you will be able to access the LUN from all hosts.
To prevent trampling the ESXi places a lock on the VMDK so that a subsequent host will not be able to power up the VM and write to that virtuak disj