1. That's not backup.

Long discussions follow those that move to externals and more. Don't repeat this lesson.

2. Copying and then telling iTunes to use the external is "well discussed". HOWEVER!!!!

3. From memory this can't work because of how iTunes uses file and folder permission on the Apple OS X so the external would have to be in Apple's native file system and that would be a PITA to get working in a Windows PC.

I'm going with no.
Bob