Adding a second drive to BTRFS

luc@kid2:~$ sudo btrfs device add -f /dev/sdc /var/backups/urbackup/
luc@kid2:~$ sudo btrfs fi show
Label: none  uuid: 9c8025a0-5351-4807-b7ac-97ffa87ededa
        Total devices 2 FS bytes used 447.00GiB
        devid    1 size 1.82TiB used 450.02GiB path /dev/sdd
        devid    2 size 1.82TiB used 0.00B path /dev/sdc