If you don't have SW15...
Optionally, if you want to independently edit the features of the mirrored part without affecting the original part, under Link, click Break link to original part.You can also break the link to the original part later by listing the mirrored part's external references and selecting Break All. Once you break the link to the original, you cannot restore it.
Thanks Glen and Thanks Kelvin,
Looks like you both were right!