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.