Created:        2026-04-12 Sun 01:30:56
Last modified:  2026-06-28 Sun 04:12:07

How to mount a partition from a former RAID1 drive

I decided to replace the NAS drives with larger ones. The replacement, including copying the old data, happened without issues.

Then I decided to check the data on the old drives using a laptop and discovered that setting up a degraded RAID1 with one drive for data access didn't work for me.

So I used a more primitive approach.

It's worth mentioning that the partitions were unencrypted and the filesystem was ext4.

I decided to find the start of the ext4 partition and mount it.

fdisk didn't give me anything helpful.

# fdisk -l /dev/sda
Disk /dev/sda: 3.64 TiB, 4000787030016 bytes, 7814037168 sectors
Disk model: EFAX-68JH4N0
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

gdisk provided more information.

# gdisk -l /dev/sda
GPT fdisk (gdisk) version 1.0.9

Partition table scan:
  MBR: not present
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with corrupt MBR; using GPT and will write new
protective MBR on save.
Disk /dev/sda: 7814037168 sectors, 3.6 TiB
Model: EFAX-68JH4N0
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): CBD7DFD7-CCFF-482A-B399-36B755C09817
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 7814037134
Partitions will be aligned on 256-sector boundaries
Total free space is 467149 sectors (228.1 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048         4982527   2.4 GiB     FD00
   2         4982528         9176831   2.0 GiB     FD00
   3         9437184      7813832351   3.6 TiB     FD00

But unfortunately, I wasn't able to mount the partition using the reported start sector.

After thinking for a while, I decided to look for the start of the partition using the ext4 magic number: ef53. I ended up with a command that successfully mounted the partition.

# mount -t ext4 -o loop,offset=$((512*(9437184+27776))) /dev/sda /mnt/

You may notice that the offset setting looks weird. It's actually the output of a small non-optimized bash script that I used to look for the start of the partition. Here is the script:

block=9437184
maxinc=100000
inc=$(( - maxinc / 2 ))
while (( $inc < $(( maxinc / 2)) ))
do
        if dd if=/dev/sda bs=1 skip=$((($block+$inc)*512+1080)) count=2 status=none | hexdump | grep  ef53;  then
          echo "$block+$inc"
  fi
  inc=$((inc+1))
done

The output can contain matches that are not indicators of the start of the partition, so I had to check all of them:

0000000 ef53
9437184+19296
0000000 ef53
9437184+27776

Finally, 9437184+27776 was the correct offset.