Hello,

following the ideas mentioned by Dan and Carl Wilhelm to add a 3rd 
harddrive to my existing RAID1 and always swap one drive to backup my 
backuppc-server I came up with the following solution:

*** How to use Software RAID1 to get a 3rd drive for offline storage ***
This has been tested with Ubuntu Server (in VMWare)

1. Existing RAID1 (/dev/md0) containing of two drives (/dev/sda1 and 
/dev/sdb1). Check if everything is fine:

        # sudo mdadm --query --detail /dev/md0

2. Connect my 3rd harddrive (dev/sdc1) to my machine and add it to the 
RAID1 as spare-drive:

        # sudo mdadm --add /dev/md0 /dev/sdc1

3. Mirror partition tables from one of the existing disks:

        # sudo sfdisk –d /dev/sda | sfdisk /dev/sdc

4. Disable one of the the two harddrives and remove it from the RAID:

        # sudo mdadm --fail /dev/md0 /dev/sdb1
        # sudo mdadm --remove /dev/md0 /dev/sdb1

5. The RAID-software will start to resync the data to the attached spare 
drive. You can check how far the process is:

        # sudo mdadm --query --detail /dev/md0

6. The harddrive that has been removed from the array can now be locked 
away to have an additional backuo in case of <INSERT-SCENARIO-HERE>


*** Further Reading ***

Most information has been taken from two locations:

1) Setting up Software RAID in Ubuntu Server
    http://tinyurl.com/24x25z

2)
    Chapter 6. Detecting, querying and testing
    http://tinyurl.com/26ua9q


*** Improvement needed - automate steps ***

After the first steps, I would like to see the following improvements in 
order to get a more userfriendly and automated offline storage backup.

1) I would like to automate the process above as soon as I attach the 
3rd harddrive to my system.

2) I would like to get an email as soon as resync has been completed.

3) Will it harm my harddrives if I use hot-swapable harddrive-cases?

4) Is it possible to sync the data to the 3rd harddrive without removing 
one of the others. This would be the best approach since I can just use 
an external disk in this case.

With the method described above I need to make two full syncs in case I 
have two not-hot-swapable-harddrives:

1. attach 3rd harddrive & add to RAID as spare
2. remove 2nd harddrive from RAID
3. resync
4. add 2nd harddrive to RAID as spare
5. remove 3rd harddrive
6. resync (even when no data has changed in between)

Hope to get some ideas how to automate everything - I know it can be 
done :-)

Best regards from Berlin

- Kurt

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