Mount a single hard disk that was part of RAID 1

Question

So I had a RAID 1 with two hard disk. One hard disk failed, then I replaced it and I reinstalled on this new hard disk a fresh Linux.

Now If I type fdisk -l I get:

root@ns354729:/mnt/sdb2# fdisk -l

Disk /dev/sda: 2000.4 GB, 2000398934016 bytes
255 heads, 63 sectors/track, 243201 cylinders, total 3907029168 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: 0xbb5259be

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        4096  1495042047   747518976   83  Linux
/dev/sda2      1495042048  1496088575      523264   82  Linux swap / Solaris

Disk /dev/sdb: 750.2 GB, 750156374016 bytes
255 heads, 63 sectors/track, 91201 cylinders, total 1465149168 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: 0x00025c91

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            4096    20975616    10485760+  fd  Linux raid autodetect
/dev/sdb2        20975617  1464092672   721558528   fd  Linux raid autodetect
/dev/sdb3      1464092673  1465144064      525696   82  Linux swap / Solaris

I would like to acces the second hard disk (sdb) so I try to mount sdb2 like this:

mount /dev/sdb2 /mnt

THis says:

root@ns354729:/mnt/sdb2# mount /dev/sdb2 /mnt
mount: block device /dev/sdb2 is write-protected, mounting read-only
mount: you must specify the filesystem type

So I tried to give:

mount -t ext4 /dev/sdb2 /mnt

and I got:

mount: wrong fs type, bad option, bad superblock on /dev/sdb2,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

And this says:

root@ns354729:/mnt/sdb2# dmesg | tail
ufs_read_super: bad magic number
VFS: Can't find a romfs filesystem on dev sdb2.
UDF-fs: warning (device sdb2): udf_load_vrs: No VRS found
UDF-fs: warning (device sdb2): udf_fill_super: No partition found (2)
XFS (sdb2): Invalid superblock magic number
(mount,18813,1):ocfs2_fill_super:1038 ERROR: superblock probe failed!
(mount,18813,1):ocfs2_fill_super:1229 ERROR: status = -22
GFS2: not a GFS2 filesystem
GFS2: gfs2 mount does not exist
EXT4-fs (sdb2): VFS: Can't find ext4 filesystem

any help?

Curious why you re-installed Linux on a fresh disk, instead of just adding it to the RAID array and letting mdraid rebuild? — derobert, Aug 03 '16 at 20:59
I did it, but the os won' boot. The system was stuck at "booting from hard disk" I also tried to reinstall grub with no luck — giò, Aug 04 '16 at 06:58

score 16 · Answer 1 · answered Aug 03 '16 at 21:03

16

You need to assemble the (degraded) RAID array, using something like:

mdadm --assemble --readonly /dev/md0 /dev/sdb2

Of course, pick a number besides md0 if that's already in use. Then you can mount /dev/md0 (or, if it is actually LVM, etc., continue down the chain).

You can, in the case of RAID1, also do this using loopback devices & an offset, but that's much more of a pain, and really is only worth attempting if the mdadm metadata has been destroyed.

answered Aug 03 '16 at 21:03

derobert

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I have tried your command, it says: `option --readonly not valid in assemble mode` if I remove --readonly it says: `/dev/sdb2 has no superblock - assembly aborted` smartctl says the hdd is fine. I am not able to recreate this damn superblock. I also followed this to recreate https://ubuntuforums.org/showthread.php?t=1947275 with no luck – giò Aug 04 '16 at 07:50
@giò No superblock? Does `mdadm -E /dev/sdb2` say the same thing? sdb is the correct disk, right? – derobert Aug 04 '16 at 15:08
Also, looking at your question, that "block device /dev/sdb2 is write-protected, mounting read-only" is rather unexpected if it's a plain ol' hard disk.... you sure that's the right disk? – derobert Aug 04 '16 at 15:10

score 2 · Answer 2 · edited Aug 20 '19 at 15:32

It looks like you have a new 2gig, non-RAID disk and you're trying to read the contents of an old 750gig RAID disk. If that's the case, you can do something like this:

ls -l /dev/md/

In our case, this returned the following:

total 0
lrwxrwxrwx 1 root root 8 Jul 26 08:30 sp:0 -> ../md126
lrwxrwxrwx 1 root root 8 Jul 26 08:30 sp:1 -> ../md127

I was then able to mount the partition I needed for reading:

mount /dev/md/sp:0 /mnt

In the case above, sp:0 and sp:1 I believe are what I named the RAID partitions at the time of creation.

score 2 · Answer 3 · answered Feb 21 '21 at 22:03

I found derobert's answer very helpful, but needed more:

From a new system that had never had an array before with the two drives attached externally via USB drive dock:

  sudo apt update && sudo apt install mdadm -y
  sudo mdadm --examine /dev/sd*
  [output of above command showed two drives that were members of the original raid array, one a PARTITION, the other the entire device]
  sudo mdadm --assemble /dev/md1 /dev/sdb1 dev/sdc --run
  mdadm: /dev/sdb1 is busy - skipping
  mdadm: /dev/md1 assembled from 1 drive - need all 2 to start it (use --run to insist).
  sudo mdadm --assemble /dev/md1 /dev/sdb1 /dev/sdc --run
  mdadm: /dev/sdb1 is busy - skipping
  mdadm: /dev/md1 has been started with 1 drive (out of 2).
  sudo mount /dev/md1 /mnt/md1
  cd /mnt/md1
  ls 
  backup lost+found pictures rsnapshot

I was then able to use rsync to copy the data from the single mounted raid member, /dev/sdc to a verified good drive on a different system.

Mount a single hard disk that was part of RAID 1

3 Answers3

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