Usually, a migration of a virtual machine from one VMware Datacenter to another is a piece of cake job. You just have to present the LUNs which contain the VM data to the new Esx servers, and you can cold migrate the VM in a matter of minutes, or even hot migrate it using a few tricks (with no downtime at all).

There’s only one thing one may not consider in the equation: the pesky Raw Device Mapping disk attached to the VM.
What is a RDM disk? It’s a LUN just mapped to the VM, without a VMFS, its pointers stored along the VM in a special VMDK file.

So you’ve just powered off your VM, browsed the datastore in the destination datacenter, and added the VM to the Inventory. As you try to power on, an error comes before you:

“Virtual disk ‘Hard Disk X’ is a mapped direct-access LUN that is not accessible.”

The RDM strikes with nonsense. You may have already checked the storage for the correct presentation, and Vsphere for visibility of the LUN: it’s all there. Why should it be not accessible?

Easier to say than to discover at first, the problem is the different LUN ID in the destination datacenter. Lun presentation, as a matter of fact, follows a numerical order and VSphere uses the specific LUN ID to map the disk to the VM.

In my case, the RDM LUN ID in the source datacenter was 23, while it was 49 on the destination.

You can check the source ID in the “Physical LUN and Datastore Mapping File” area in the RDM disk properties in the VM Settings. There are many ways you can check the correspondence in the destination datacenter, via both Vsphere and command line. In Vsphere, the easier way is to check in Configuration > Storage > view: Device and sort by the size of the disk: mine was 1.7TB, easy to spot. If you have a trickier, more common size, you have to identify and compare the UID of the disk with commands such as “esxcli storage core path list”.

So, there are ways to solve this problem but VMware’s proposed solution is actually the less favorable, as they ask to remove the LUN and present it with the correct ID. But in my case ID 23 was already in use.
It would be better to just map the RDM to the VM with the new ID, but VSphere won’t even allow to see the RDM from the Add Disk Wizard: those already initialized are filtered by default.

What we’re going to do is disable the LUN filtering so we can attach the same LUN with the destination ID.
In the vSphere Client, select Administration > vCenter Server Settings > Advanced Settings.
Then add the following key and value: config.vpxd.filter.rdmFilter; false
Click Add, and click OK.
Now, in the VM properties, first of all detach the source ID LUN from the VM, then click Add > Hard Disk > Raw Device Mappings and select the correct LUN.

The RDM is now properly attached, and the VM will finally boot on the destination datacenter.