To make sure I understand this correctly.
You are talking about 2 ServerFarms. Does that mean two separate vCenter Server environments or multiple DataCenters in a single vCenter Server environment?
Since the LUN is used as an RDM, can you confirm you used the same .vmdk RDM mapping file on the second ServerFarm, i.e. didn't create another mapping file? This is important regarding locking and data integrity. Only a few operating systems allow simultaneous access to the same disks/partitions.
Which OS does have access to this RDM? Is it configured as shared or shared-nothing (e.g. MSCS)?
André
