Why do I have multiple drives in my backup system image?

Why do I have multiple drives in my backup system image?

I have a drive which has 2 partitions. One is where the OS is installed, the other is a data (but not libraries) drive. When I try and create a backup using the built in tool, it wants to include both partitions in the system image. Why does it do this? If I move the os to a separate drive will I be able to back up just this data?