Storage vMotion with RDMs on VMs

when you are performing Storage vMotion operations on a VM with at least one RDM and you want/need to convert your virtual disks from thick to thin (or vice versa) during the migration, you need to perform two (2) separate Storage vMotion operations:

1. First, you’ll migrate only the virtual disks attached to the VM. You’ll use the Advanced button when selecting the datastore so that you can leave the RDM alone for this migration. You’ll then select the appropriate format (“Thin Provisioned Format” or “Thick Format”) for the virtual disks on the target datastore and proceed with the Storage vMotion operation.

2. When the first Storage vMotion concludes, you’ll then perform a second Storage vMotion operation to migrate only the RDMs. Again, you’ll use the Advanced button when selecting the datastore and choose to move only the RDM. For format, select “Same Format as Source”, as this is the only option that preserves the RDM as an RDM. If you are migrating a virtual mode RDM and choose either of the other two options, your RDM is converted to a virtual disk and cannot be converted back.

At this point, your RDM-equipped VM has been migrated to a new datastore, the virtual disks have been converted from thick to thin (or vice versa), and your RDMs have been preserved as RDMs.

Also please check this blog: http://blogs.vmware.com/vsphere/2012/02/migrating-rdms-and-a-question-for-rdm-users.html