# mdadm --create --verbose /dev/md3 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1
mdadm: /dev/sda1 appears to contain an ext2fs file system
    size=6305480K  mtime=Fri Apr 25 16:16:56 2003
mdadm: size set to 120623936K
Continue creating array? yes
mdadm: ADD_NEW_DISK for /dev/sda1 failed: Device or resource busy