Mount a single drive from a two-disk Synology SHR RAID 1 group, on Pop!_OS (or another Ubuntu-like OS)

A number of years ago, I upgraded from my original DS214se NAS to a more powerful DS218+, donating the DS214se to my parents to act as a Time Machine backup drive and video server.

My parents live in the most beautful corner of Herefordshire. The one down-side of this location is very occassional blips in power supply (all the rural dwellers reading this will be nodding right now). Usually this doesn’t cause an issue. But over Christmas my parents told me they’d noticed the DS214se was stuck in some sort of loop, attempting to boot up again and again, but never completing. I figured it had improperly shutdown at some point before, and was now stuffed.

I tried all the usual troubleshooting, nothing worked.

It probably needs a reset and a clean pair of disks. But before I do that, I wanted to get a copy of everything on the existing disks, just in case. I knew this wouldn’t just be a case of plugging the one of the SATA drives into my PC, though, because these drives had been configured as an SHR (“Synology Hybrid RAID”) RAID 1 group.

Synology publishes a guide on mounting SHR disks on an Ubuntu PC, but when I tried it, I got stumped by an error message from mdadm:

root@pop-os:~# mdadm -AsfR && vgchange -ay
mdadm: No arrays found in config file or automatically

I note Synology’s guide says you need all of the disks from your SHR RAID group – but I only had one SATA cable, and I also quite liked the idea of leaving one disk entirely untouched, just in case my attempts to mount the disks also fried them. I wanted to mount just one of the two disks.

(Theoretically, this should work fine, since a two-disk Synology NAS in SHR mode will use RAID 1, which simply mirrors all the same content onto both disks – it should be possible to mount any one of those disks individually.)

But how do you do it?

What you’ll need

An Ubuntu-like computer/environment (I used my Pop!_OS gaming PC, but Synology’s guide includes links to setting up an Ubuntu Live USB drive if all you have is a Windows or Mac device)
Any one of the two disks from your Synology’s SHR RAID 1 group
A way to connect the disk to your computer (I used a USB-to-SATA cable, or you could plug the disk directly into your motherboard – note, 3.5 inch disks will need an external power connection)

As mentioned in Synology’s guide, you’ll need to install mdadm and lvm2 (my Pop!_OS system already had these installed):

$ apt-get update
$ apt-get install -y mdadm lvm2

Mounting the disk

Get into an interactive root shell:

$ sudo -i

Plug in and power up the SATA disk. With any luck it’ll spin up, and Ubuntu will start reading it.

At this point, you could try running the commands that Synology suggested – although, as I say, these didn’t work me:

$ mdadm -AsfR && vgchange -ay

If it doesn’t work, instead try running lsblk to see what partitions are on the disk:

$ lsblk

In my case, this was the output:

NAME            MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINTS
sda               8:0    1     0B  0 disk  
sdb               8:16   0   1.8T  0 disk  
├─sdb1            8:17   0   2.4G  0 part  
├─sdb2            8:18   0     2G  0 part  
├─sdb3            8:19   0     1K  0 part  
└─sdb5            8:21   0   1.8T  0 part  
  └─md127         9:127  0     0B  0 md    
zram0           252:0    0    16G  0 disk  [SWAP]
nvme0n1         259:0    0 953.9G  0 disk  
├─nvme0n1p1     259:1    0  1022M  0 part  /boot/efi
├─nvme0n1p2     259:2    0     4G  0 part  /recovery
├─nvme0n1p3     259:3    0 944.9G  0 part  
│ └─cryptdata   253:0    0 944.9G  0 crypt 
│   └─data-root 253:1    0 944.8G  0 lvm   /
└─nvme0n1p4     259:4    0     4G  0 part  
  └─cryptswap   253:2    0     4G  0 crypt [SWAP]

nvme0n1 is my PC’s M.2 SSD drive (with the encrypted root partition on nvme0n1p3 and the Pop!_OS recovery partition on nvme0n1p2).

zram0 is the system swap disk. And sda is empty.

So that just leaves sdb with a total capacity of 1.8T (1.8 terabytes) – looks like my SATA disk! The sdb5 partition, occupying almost all of that 1.8 TB drive, is the one with all the data on, and the one we’ll want to mount. But how do we mount it?

It turns out you can force mdadm to recognise just one of the two RAID 1 disks:

$ mdadm --assemble --run /dev/md0 /dev/sdb5 --force

/dev/md0 there is just a new device name for mdadm to assemble the RAID group into – you can pick whatever you like, but it seems /dev/md0 is traditional. And /dev/sdb5 is the partition we identified in the previous step.

Once you’ve run that, you’ll hopefully get the golden response:

mdadm: /dev/md0 has been started with 1 drive (out of 2).

(Top tip: if at any point you get into a bit of a mess, and run mdadm on a non-RAID partition, giving you mdadm: [blah] is busy - skipping errors, you can tell mdadm to stop with mdadm --stop --scan.)

So, we’ve got the RAID group assembled. Next we need to identify the logical volumes inside it. If you run lvs or lvscan you’ll see all the available volumes on the computer:

$ lvscan
  ACTIVE            '/dev/data/root' [<944.85 GiB] inherit
  WARNING: PV /dev/md0 in VG vg1000 is using an old PV header, modify the VG to update.
  ACTIVE            '/dev/vg1000/lv' [1.81 TiB] inherit

(You’ll also get a warning about “an old PV header”, which you could fix with vgck --updatemetadata vg1000 but I decided not to, because everything seems to work fine without it, and I want to avoid changing anything on the disk unless I absolutely have to.)

Now we know what the volume group is called (vg1000) we can activate all of the logical volumes inside it:

$ vgchange -ay vg1000
1 logical volume(s) in volume group "vg1000" now active

And our volume is ready to mount!

Create a mountpoint for the volume (this could be a new directory anywhere on your computer – traditionally mountpoints go into /mnt, so I created mine at /mnt/hd1):

$ mkdir /mnt/hd1

And then mount the volume, in read-only mode:

$ mount /dev/vg1000/lv /mnt/hd1 -o ro

/dev/vg1000/lv is the logical volume path from the lvscan output above, while /mnt/hd1 is the mountpoint we created earlier, and -o ro sets the “read-only” option, so we don’t accidentally modify anything on the disk.

You can now open up /mnt/hd1 in the file explorer, and copy stuff from it!

When you’re done, you can unmount the volume:

$ umount /mnt/hd1

Then deactivate the logical volume group:

$ vgchange -an vg1000

And tell mdadm to stop:

$ mdadm --stop /dev/md0

With the filesystems unmounted, you can “power off” the hardware device itself (to stop the disk spinning) with:

$ udiskctl power-off -b /dev/sdb