Say you wanted to create a RAID1 device but didn't have all your devices ready. Here is how you could create a degraded RAID1 array and then add the second device at a later time. For these examples /dev/sda1 is the first device which will become our RAID and /dev/sdb1 will be added later.
First step is easy, create the RAID array. Our array will be /dev/md0. The "-n 1" option tells it that just a single device will be used and "-f" is required to use such a non-standard option.
$ mdadm --create /dev/md0 -l raid1 -f -n 1 /dev/sda1