Hyper-V Program Manager
If you are having problems using the VHDMount tool that is included with Virtual Server 2005 SP1, something to do to help you to figure out what could be going wrong is to run:
Before using VHDMount. Then you will see tracing information about what is going on 'under the covers' of VHDMount.
Sorry to jump in here, but you might want to warn people of the following: http://support.microsoft.com/kb/937366 (copying large files can hang the host machine). It's probably best to avoid using a production machine.
Fortunately, if this happens, the undo disk is left, which is a valid VHD, so lost files may be recoverable.