I just made a fresh install of Ubuntu 12.10 on a new SSD (/dev/sda). Now I would like to add two 1TB drives (/dev/sdb /dev/sdc) in RAID 1 but I'm pretty clueless on how to do it.

This is what I came up with (but didn't work):

First created partitions on the new drives:

sudo fdisk /dev/sdb

created primary partition and write table to disk

After that:

sudo mdadm --create --level=1 --name=raidarray --raid-devices=2 /dev/sdb /dev/sdc

However this outputs:

mdadm: device /dev/sdb exists but is not an md array.

Can somebody point me in the right direction on how to do this?

One bonus question: Would it be possible to reserve a part (same size of SSD) of one HDD and create a RAID 1 with the SSD and part of the HDD. And if so, how and can I keep my installation?

2 Answers 2


The default command should look like this:

mdadm --create --verbose /dev/md0 --level=1 /dev/sda1 /dev/sdb2

You are not adding a complete disk, you should add a device (use /dev/sdb1 (or2), not just /dev/sdb) as far as I know. The target name (/dev/md0) was also missing.

I always use this cheatsheet: http://www.ducea.com/2009/03/08/mdadm-cheat-sheet/ :D


You have not specified the name of the resulting raid device. As such, mdadm is assuming that you're trying to create a RAID device ad /dev/sdb and then failing because /dev/sdb already exists.

You should modify your command:

sudo mdadm --create --level=1 --name=raidarray /dev/md0 --raid-devices=2 /dev/sdb /dev/sdc

/dev/md0 is the name of the new RAID device you're going to create.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .