This tool saved me lots of time. So I thought I point you to it:

http://code.msdn.microsoft.com/vhdtool

Among some other nice features, my favorite is repair

        Repairs a broken Hyper-V snapshot chain where an administrator
        has expanded the size of the root VHD.  The base VHD will be
        returned to its original size. THIS MAY CAUSE DATA LOSS if the
        contents of the base VHD were changed after expansion.
 

Cheers

Robert