RAID Rebuild
OS Disk
Replace /dev/sda and /dev/sdb if required !!!
Wipe New Disk
wipefs -a /dev/sdb
Copy Partition Table from other Bootable-Disk to New Bootable-Disk
sfdisk -d /dev/sda | sfdisk /dev/sdb
NOT DONE YET, now replace the "broken" device with the new one using btrfs tools and scrub after that one time maybe.