Existing setup: host1 and host2, ESX 4.0, 2 HBAs each. lun1 and lun2, 2 LUNs belonging to the same RAID set (my terminology might be sketchy here). This has been working just fine all along. I added host3, ESXi 4.1, 2 HBAs. If I view Configuration / Storage Adapters, I can see that both HBAs see both LUNs, but if I view Configuration / Storage, I only see 1 datastore. host1/2 can see both LUNs and I have VMs running on both too. I have rescanned, refreshed and even rebooted, but host3 refuses to acknowledge 1 of the datastores. Does anyone know what's going on? Update: I re-installed the host with ESX (not i) 4.0, same version as the existing hosts and it's still not recognising the vmfs. I think I'm going to SVmotion everything off that datastore then format it. Update2: I've created the LUN from scratch and the problem gets even weirder. I've presented the LUN to all 3 hosts, and I can see the LUN in the vSphere client's Configuration / Storage Adapters section on all 3 hosts. If I create a datastore on the LUN via the Configuration / Storage section on host1, it works fine and I can create an empty folder via datastore browser, but the datastore is not seen by the host2 and host3. I can use the Add Storage wizard on host2 and it will see the LUN. At this point the "VMFS Label" column has the label I gave with "(head)" appended. If I try the Add Storage wizard's "Keep the existing signature" option, it fails with an error "Cannot change the host configuration." and a dialog box that says 'Call "HostStorageSystem.ResolveMultipleUnresolvedVmfsVolumes" for object "storageSystem-17" on vCenter Server "vcenter.company.local" failed.' If I try the Add Storage wizard's "Assign a new signature" option on host2, it will complete and the VMFS label will have "snap-(hexnumber)-" prepended. At this point its also visible on host3, but not host1. I have a similar setup in a different datacenter which didn't give me all this trouble.