Unmountable: No file system

You could unassign the disk from the array, and restart the array. UnRaid would simulate the disk. You could then zero the simulated disk. This would update parity without writing to the data disk. After all done you could physically remove the disk. But doing the new config, preserving drive assignments, unassigning the drive you want to remove from the array, and then starting would trigger a parity build and functionally do the same thing. 

Assigning a data disk to the parity slot and starting the array is deadly unless you detect it right away. And even then, check tools may not recover effectively. The old adage "measure twice cut once" comes to mind. When you are doing whole disk operations, you need to be particularly careful and double check everything before taking the plunge.

In the old days I remember the "Are you sure?" prompt from Dos. It was so automatic to hit Y and enter. After getting burned a couple times, I trained myself to literally take my hands off the keyboard when I saw it, and recheck all my prior steps before hitting the Y. (The same reflex kicks in today when I see warnings to confirm this or that.) This saved my bacon more than a few times over the years. We have to realize that most data loss is due to user error. No number of parities protect us. We are the enemy!