How can one do this?

I figured out, using JMFS, how to make a 2tb internal.

I know that to make an external you need 15 partitions on the internal drive, so I added
a 'Apple_Free' as 15

But then, It won't add the expander drive. It tries, reboots and tries again. Never succeeding

I know it can be done, Weaknees does it

Any ideas?