Sorry, i forgot "/mnt/disk1" created after Array starts.
You are absolutely right. i tried according to your steps and it worked well. Thanks.
PS:
"-f" is needed when typing "btrfs device add /dev/sdb1 /raid" in my case.
Good idea.
But why not try "btrfs device add /dev/sdb1 /mnt/disk1", is there anything not compatible with default Array running?
Or just add sdb1 to Array as disk2, Try "btrfs device add /dev/md2 /mnt/disk1".
i have no free disk here, can someone do me a try?