I'm also noticed this
Using System Manager is the right tool if you have a planned DR test, you can do it properly (Update,quiesced, break)
But in our case, you power off source filer before running test
It's like "real" DR
So you don't need or will be able to quiesced .........
And there we find all the utility ,old friend CLI, just snapmirror break vol_name
After our DR test, when source filer comes back online , you will be able to use SystemManager for doing by exemple reverse sync or whatever you want