Creating a fixed size virtual hard disk in Hyper-V can be a timely process. It is because the contents of the empty file are overwritten to ensure no sensitive data from the underlying “host” file system are accidentally revealed to the child partition or guest operating system. That’s not such a problem on the current generation of Hyper-V in a clustered environment if your shared storage LUN’s are created specifically for the virtual machines (and immediately deleted afterwards). If this is the case then you can use the tool that is mentioned in John Howard’s post. This tool can create an unsecured VHD in seconds.
However, you should not use this on a reused file system or on a shared purpose file system. I’m also thinking it probably shouldn’t be used on the new Cluster Shared Volume in Windows Server 2008 R2 Hyper-V. Why not? What if you create a VM with sensitive data and then remove it. You create a new VM with an unsecured VHD using the tool. Now the unused contents of that VHD can be scanned by someone who’s logged into it to pick up contents of the old VM’s VHD. Ouch!