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

You can confirm that your device is working through /proc/mdstat:

$ cat /proc/mdstat
md0 : active (auto-read-only) raid1 sda1[0]
4881700 blocks super 1.2 [1/1] [U]

Now we need to add this device to /etc/mdadm/mdadm.conf, otherwise the system won't see it automatically after the next boot. First we have to find out the UUID that was assigned to the array:

$ mdadm --detail /dev/md0 |grep UUID
UUID : 7fc1afb1:e68bef6b:8972b79f:1bd36cd5

Add a line like this to your mdadm.conf:

ARRAY /dev/md0 metadata=1.2 uuid=7fc1afb1:e68bef6b:8972b79f:1bd36cd5

That's all it takes to create the degraded array. Now comes the fun when you want to expand it. First step is to grow the array to have 2 devices instead of just one with this command:

$ mdadm --grow /dev/md0 -n 2

You can confirm it's there with mdadm --detail:

$ mdadm --detail /dev/md0
/dev/md0:
...
State : clean, degraded
Active Devices : 1
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

Number Major Minor RaidDevice State
0 8 19 0 active sync /dev/sda1
1 0 0 1 removed

All that's left is to add the new drive to the array where Linux will automatically put it into place where the missing disk is.

# mdadm --manage /dev/md1 --add /dev/sdb1
mdadm: added /dev/sdb1

# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb1[1] sda1[0]
29221139 blocks super 1.2 [2/1] [U_]
[>....................] recovery = 1.7% (506496/29221139) finish=8.5min speed=56277K/sec