Wednesday, February 18, 2015

How to mount a single disk that was part of RAID 1

I screwed up and deleted the folder /etc of our main computer (samba server, dhcp, router, etc.). Terrible mistake...

The positive thing is that it was an old Ubuntu 8.04 and we wanted to update it.
So here was the time to do it.

On the machine, there was 2 hard drives in RAID 1 (mirror). So we removed them and put 2 new drives. We installed Ubuntu 14.04 and all the necessary packages.

Then it was time to copy data from the old disks to the new ones.
I didn't want to lose any more data, so I wanted to connect only 1 hard drive.

The problem is that this hard drive was part of a RAID, it is not a normal hard drive that we can mount as usual.

If you try, you will get error like:

mount: unknown filesystem type 'linux_raid_member'
mount: /dev/sdc2 already mounted or /mnt/recovery busy


Here are the steps to do:

1)  create a mount point
mkdir /mnt/recovery

2) list the disk information
fdisk -l

Disk /dev/sdc: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders, total 1953525168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000abbde

Device Boot      Start         End      Blocks   Id  System
/dev/sdc1   *          32    62500863    31250416   fd  Linux raid autodetect
/dev/sdc2      1937904885  1953520064     7807590   fd  Linux raid autodetect
/dev/sdc3        62500864  1937899519   937699328   fd  Linux raid autodetect

I want to mount /dev/sdc3. The important information that we need in the next step:
  • sectors: 512
  • start: 62500864

3) finally run these commands:
$ losetup --find --show --read-only --offset $((62500864*512)) /dev/sdc

$ fsck.ext3 -n -v /dev/loop3
e2fsck 1.42.9 (4-Feb-2014)
Warning: skipping journal recovery because doing a read-only filesystem check.

$ file -s /dev/loop3
/dev/loop2: Linux rev 1.0 ext3 filesystem data, UUID=dbf4-...-9a98a (needs journal recovery) (large files)

$ mount -t ext3 -o ro,noload /dev/loop3 /mnt/recovery
note: I'm using loop3, but it can be a different number in your case.

And that's it! you can now access your files.

If you have an error like:
mount: wrong fs type, bad option, bad superblock on ....
It probably means you have entered a wrong offset.

In case of error, use the following command to see the log:
dmesg | tail

Another solution on a computer without any RAID

On my desktop, I have no RAID disk, so I could run the following command:

mdadm --examine --scan

And that's it!

Here are 2 links that helped me a lot to find the solution!

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