I discovered that there is a very important difference between 'sgdisk -R /dev/sda /dev/sdb' and 'sgdisk /dev/sda -R /dev/sdb'

the latter replicates the GPT disk layout from one disk to another. ideal for rebuilding a software RAID 1 array.

the former creates a blank GPT layout on the non-failed disk, thus destroying the array. less than ideal for rebuilding a software RAID 1 array.

So yeah. There is no confirmation or error checking. thank goodness for backups.