RAID 1 Setup – How to Setup RAID 1 in Linux Mint 13

Yes, you can use chroot to install mdadm:

sudo -s
for f in sys proc dev ; do mount --bind /$f /mnt/$f
chroot /mnt
apt-get install mdadm

Replace /mnt with wherever you mounted your root filesystem.

Ouch!

All the searches I done showed how to reassemble the array assuming 1 bad drive.

That is because RAID5 will not work with more than one failed drive. You can not guarantee to recover all data with two missing drives. In fact, if both drives are fully inaccessible recovering it will fail. The data simple is not there anymore.

Two notes:

1. I wrote fully down. As in dead disk, drive removed from system. Not just a single bad sector.
2. The usual rant that RAID is not a backup. If RAID fails you just have to keep the system up till 5PM, backups the files changed since the last backup (using incremental backup) and then you can either try a lengthy rebuild or rebuild the RAID and restore from backup. Obviously as a home user you do things slightly different, but the same problem persists when doing a RAID5 rebuild and getting an URE.

( Also see This canonical post Serverfault and this post on S.U. and this post on S.U. )

In your case I see these options:

1. Send the drives in to a very expensive data recovery lab. These things are really expensive.
2. Give up and restore from an old backup.
3. Try to mount the RAID arrays with two drives missing.

Before you try option 3: Make a backup of the drives. Place them in another system and copy the drives with dd or ddrescue. Keep those images. If things fail you can restore to the current situation from these. (read: things will not get worse).

You then can try to recover either from the NAS, or from the system where you stored the images. Make a working copy of them and use the loopback device. If you have sufficient diskspace then this is the preferred way, though you would need a place with twice the free diskspace of your entire NAS.

Next read this rather lengthy blog at http://blog.al4.co.nz/2011/03/recovering-a-raid5-mdadm-array-with-two-failed-devices/.

The essential steps in it are:
mdadm --create /dev/md1 --level=5 --raid-devices=5 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1 missing

That would mark drive 5 as missing. I selected that one because I have no idea what state it is in after a partial rebuild.

With a bit of luck you can now mount it as a degraded array. Copy all data off it, then delete the array and rebuild. It may hang during the copying of data. In that case reboot, skip a few files and continue. It is far from perfect, but if recovery is to expensive and you have no backups then this might be the only way.