If your original csys has parent references that are not replicated in your new model, then copy from diff model won't work (there will be no corresponding new refs to use).
If you want a c-sys without external references, your best bet is sometimes the following method:
- Make sure both models (source and target) have a common csys location somewhere (we'll call this the origin)
- Create a transform measurement in the source model from the origin csys to the csys you wish to copy. Then hit the info button and save this information to a file (probably named info.trn or something similar).
- Open the target model and create a csys Using File reference the origin csys and the file you created above.
This will result in a csys in the right location and orientation with no external references and without the need to create additional reference geometry to support it.