I had this same problem on a friend's Sony laptop. I found a solution in this thread: http://social.technet.microsoft.com/Forums/en-US/w7itprogeneral/thread/ac789e92-6283-4e5a-bcf8-a8119ab4cc92
The specific solution is this: "Went into Control Panel -> Power Options -> Change Plan Settings -> Change Advanced Settings. Under USB Settings changed USB selective suspend to Disabled. You guessed it - system image backup now works!"
Don't know if this will work for you.
Ivan.