mkfs.btrfs -m raid1 -d raid1 /dev/sda1 /dev/sdb1

$ sudo btrfs filesystem show
Label: none  uuid: 607b4153-7aa9-444d-bc15-c5fe9038f255
    Total devices 2 FS bytes used 28.00KB
    devid    1 size 1.82TB used 2.03GB path /dev/sdc
    *** Some devices missing
Label: 'Max'  uuid: b6647427-9f27-4157-b47b-77f74054b885
    Total devices 1 FS bytes used 28.00KB
    devid    1 size 931.49GB used 2.04GB path /dev/sdc1        // first one
Label: 'Moritz'  uuid: d0eaf97d-249e-4b7c-88a5-b60cc2d489d9
    Total devices 1 FS bytes used 28.00KB
    devid    1 size 931.49GB used 2.04GB path /dev/sdc2        // second one

$ sudo mkfs.btrfs -L RAID-Datensicherung -m raid1 -d raid1 /dev/sdc1 /dev/sdc2

$ cd /media
$ sudo chown julien:julien RAID-Datensicherung
$ sudo chmod 700 RAID-Datensicherung