There are some really cool new features in Windows Server 8 - especially surrounding Hyper-V and storage. Aidan Finn wrote a really good article detailing some of the new Hyper-V features, but I wanted to bring your attention to one in particular - storage live migration. The details are toward the end of the article, but here are the highlights:
"Say you want to move a VM's storage from location A to location B. The first step done is to copy the files.
IO operations to the source VHD are obviously continuing because the VM is still running. We cannot just flip the VM over after the copy, and lose recent actions on the source VHD. For this reason, the VHD stack simultaneously writes to both the source and destination VHDs as the copy process is taking place.
Once the VHD is successfully copied, the VM can switch IO so it only targets the new storage location. The old storage location is finished with, and the source files are removed. Note that they are only removed after Hyper-V knows that they are no longer required. In other words, there is a fall back in case something goes wrong with the Live Storage Migration.