Can't change MBR

When my Winclone 2 comes to the end of a multiple hour transfer process of Windows 7 from an external drive where it was saved by Winclone to an another external drive that has a bootcamp partitiion created on it, I get a “cannot change MBR” message and the bootcamp partition disappears from my mac. I have done this twice now, once with the helper request confirmed at the end of the load and once after cancelling the request. Same result.