I have been moving some of my Hyper-V installs from 2008 R2 to 2012. Because of the large number of VHD’s attached to a particular VM, I elected to disconnect the disk from the old host and connect them to the new host, and then do an import.
I did this on a few smaller instances and it worked fine, but on this one I kept getting an error every time I tried to start the VM. The error was “Failed to power on with error The process cannot access the file…” and the path to one of the many VHDs.
In the end, I removed the antivirus. I believe what was causing the problem was a time out trying to access all the VHDs and the antivirus was slowing it down too much.