In case it helps anyone else, I wanted to resurrect this old thread with a me too and some additional information. In my case, I was working with a copy of 5969303 as a fresh install that doesn't have internet connectivity yet, nor is it hooked to vcenter yet. I wanted to test that some iSCSI LUN snapshots were replicating successfully from one data center to another, so all I wanted to do was boot this copy of 6.5 5969303, mount the snapshot LUN on the local array, boot a VM. No matter what I did in the GUI, or CLI, I could not mount the iSCSI LUN. It would show as a storage device in the web interface, but I could not do anything with it, and the actions only allow me to add a new vmfs datastore which would partition and format it. The GUI did reflect the correct partition size. esxcfg-volume -l showed nothing.
I got some internet for this host and updated to 7388607. Reboot. esxcfg-volume -l and now it shows as available; used -M and successfully mounted.
Perhaps there's some VMFS versioning issue that affects certain builds?