New
#11
Bootmgr is missing - Fix
Have you tried using the manual method outlined in step 4 of this tutorial? I know you are worried about making changes and not being able to boot afterwards, so before attempting anything else, you should make a system image of both drives, if you haven't done so already. That way you can restore things to the way they were if something should go amiss. I use Macrium Reflect also, as whs suggested. Also please remember to disconnect the other drive from your system when making any changes, and when you disconnect the drive, shut down your system to do so. You may be aware of all that, but I just want to be sure.