How can I copy a variable between projects?

How can I copy a variable between projects?

Can anyone tell me how to copy a variable between projects in the same workspace?
I have a master project and several sub-projects and when I try to make a copy of a variable from one sub-project to another I just get this message:
Project overlapping copying of variable not possible!

This is a migrated post! Originally posted on 11.07.2014 by user mjackson. Please be aware that information can be outdated.