RAID 1

Copy partition table from /dev/sda to /dev/sdb and extend btrfs to RAID1 

sfdisk -d /dev/sda > part_table
sfdisk /dev/sdb < part_table
btrfs device add /dev/sdb1 /
btrfs balance start -dconvert=raid1 -mconvert=raid1 /

btrfs boot issues

Configure grub to use 4.9.0-12 kernel in /etc/default/grub: 

GRUB_DEFAULT="1>4"

Selects *2nd* menu option ("Advanced settings"), then *5th* submenu option.

Replace Failed BTRFS Disk

Identify the failed disk then yank it

btrfs device stats /   # show which disk has errors
mount   # observe mount option 'degraded'
btrfs filesystem show /

btrfs filesystem show output: 

 Label: none  uuid: 85f1e251-bb5b-45eb-9107-b804b9087464
        Total devices 2 FS bytes used 17.89GiB
        devid    2 size 111.79GiB used 80.02GiB path /dev/sdb1
        *** Some devices missing

Missing disk is devid 1.

Hot Swap the new disk

echo ';' | sfdisk /dev/sdi  # partition

# replace devid:1 with disk at /dev/sdi1 for mountpoint /
btrfs replace start -r 1 /dev/sdi1 /

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