Unbootable system due to grub-probe failing while root pool is resilvering

[nickcrabtree]

Attempting to replace a failed disk in a 3-way mirror of my root pool. This is on bare metal, a production server which has been running Ubuntu 16.04 root-on-zfs for 18 months. Following these instructions, I got the error message below at step 5

grub-probe: error: unknown filesystem.

It appears that grub-probe is probing /dev/sdb, which happens to be the disk which is being resilvered.

Output from grub-probe -vv, zpool status and ls -l /dev/disk-by-id attached

grub_probe_vv.txt
disk-by-id.txt
zpool_status.txt