We got a question from a customer today about disk expansion in combination with RDMs in ESX. The customer had a virtual machine that used a vmdk disk for the C: drive and a raw SAN LUN as a data disk (using RDM). The customer had expanded the LUN to add extra space. However, after booting the VM, the added free space was not visible in the Windows disk management snap-in.

This behavior is expected. What you need to do is to run vmkfstools -X <newsize> on the vmdk file that is used for mapping the disk to a SAN LUN. The free space will then be visible to Windows, allowing you to run diskpart to expand it.