I have posted this earlier and thought it was solved but this keep hunting me!!
I have a host server running Server 2008 R2 FULL with the Hyper-V role enabled.
I have installed two virtual servers.
After restarting the host only the first virtual server is starting.
When the second try to start Hyper V manager returns an error:
"An error occured while attempting to start the selected virtual machine.
Failed to open attachment B: Error: The process cannot access the file because it is being used by another process."
Attachment B: is the physical letter of the dvd drive on the host server.
This used to be drive letter F: but after posting this Q the last time I changed the drive letter which caused the machine to start!!