Bootmgr is missing error is to be expected when booting after changing the Active flag.
As stated you need to boot into the Windows 7 DVD or Repair CD to run Startup Repairs x3.
Booting into a disk is not affected by Bootmgr is missing
at all which refers only to the HD set first to boot which still needs System boot files.
Moving Boot files using EasyBCD is fine but as I said earlier you may not be able to boot the other HD now and it does not write the System Recovery Options
to F8 Advanced Boot TOols menu, which is one of the things Startup Repair does.
You can however add the other OS if still needed using EasyBCD.