I work at a school, and the teacher of our technology class made the image and asked me to clone it. Apparently Windows 10 automatically becomes default bootloader. So in my scenario, Windows 7 was on disk1, the teacher installed Windows 10 on disk2, Windows 10 detected the boot record and just modified it? I have no idea, I can't quite wrap my head around it.
To add to this, I got this working, not sure it should be working this way...but it is. I cloned the entire 2nd disk (/dev/sdb to /dev/sdb) I then cloned /dev/sd1a (which is the partition with the bootloader) to disk1 on the destination machine, and both Operating Systems now boot. It did not in fact wipe the entire drive. I know you said it should, and I believed it would, but it wasn't working anyway so I thought I'd try. Long story short, both drives are functioning properly. I guess I should add all the destination hard drives are set up the same way regarding partions.
I'd like to understand why, but at the same time I'm just glad it's working.