root@baby ~ # mdadm --create /dev/md3 --auto md --level=1 --raid-devices=2 /dev/sdc1 /dev/sdd1
mdadm: cannot open /dev/sdc1: Device or resource busy

but:

root@baby ~ # df -h
Filesystem      Size  Used Avail Use% Mounted on
udev             16G     0   16G   0% /dev
tmpfs           3.2G  8.9M  3.2G   1% /run
/dev/md2        204G   28G  167G  15% /
tmpfs            16G     0   16G   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs            16G     0   16G   0% /sys/fs/cgroup
/dev/md1        488M   55M  408M  12% /boot
tmpfs           3.2G     0  3.2G   0% /run/user/0

so it's not mounted. why cant I create a raid array from it?