Have you confirmed your current SEM is using MBR? Do you know what version you first deployed on?
A quick check you can do is go into Edit Settings on your virtual appliance in your VMware vCenter or Hyper-V Manager and see how many hard disks your SEM has. If it has Hard Disk 1 and Hard Disk 2, you're safe. If you only have Hard Disk 1, then that means it was deployed prior to 6.4 and you're looking at a redeployment to expand beyond 2 TB.
Thanks,
Jason