The error above happened to me today when setting up a new Virtual Machine on a brand new HyperV Server running on Windows Server 2008 R2 Exterprise.
Microsoft Support have a Knowledge Base article on this here – http://support.microsoft.com/kb/2249906
Basically, the fix is to copy the Virtual Machine ID and run the icacls command from an elevated command prompt.
eg. icacls “E:\VMs\VMName\Disk0.vhd” /grant “NT VIRTUAL MACHINE\5FC5C385-BD98-451F-B3F3-1E50E06EE663”:F