Q: Will shared virtual hard disk (VHDX) work with Windows Server 2008 and Windows Server 2008 R2?

A: Shared VHDX is supported for guest OSs running Windows Server 2012 and Windows Server 2012 R2 (which should be running the Windows Server 2012 R2 Hyper-V Integration Services). Within the guest OS, the shared VHDX looks like shared SAS storage, which means there's no actual reason it wouldn't work with Windows Server 2008 and Windows Server 2008 R2. I actually tried it with Windows Server 2008 R2, running the Windows Server 2012 R2 Hyper-V integration services, and it worked fine. However, this wouldn't be a supported scenario.