Seriously, I quickly went to the physical office, brought down the machine 10 minutes to physically install two hard drives and clean things a bit; made sure #freebsd saw them with their full storage capacity, and left.
Hours later I added the new devices to the mirror, waited for things to be 'copied over' (resilver), checked again at some point, brought the old drives offline, checked again after a couple days, removed the drives from the pool and resized it.
