Synology volume crashed or storage pool degraded? What it means and what to do

Synology’s Storage Manager uses a handful of short status words to describe the health of your NAS: Healthy, Warning, Degraded and Crashed. They sound similar, but they mean very different things for your data, and the right next step for one can be the wrong step for another. This guide explains what each warning means, what is safe to do yourself, and when to stop and get help.

Storage pools, volumes and drives

In DSM (Synology’s operating system), your drives are grouped into a storage pool, which uses SHR, SHR-2 or a standard RAID level to protect against drive failure. On top of the pool sits one or more volumes, formatted with Btrfs or ext4, which hold your shared folders. DSM reports a status for each layer, and for each individual drive, so it is worth looking at all three in Storage Manager before doing anything.

“Warning” on a drive

A drive marked Warning is still working, but DSM has seen something it doesn’t like: a rising count of bad sectors, a failed health test or a drive that has reached a usage threshold. Your pool is usually still Healthy at this point. This is the best moment to act: make sure your backup is current, then plan to replace that drive while every other drive is still fine.

“Degraded” storage pool

Degraded means the pool has lost a drive but is still within what its redundancy can cope with. One failed drive in an SHR-1 or RAID 5 pool, or up to two in SHR-2 or RAID 6, leaves the data accessible but with no safety margin left (or less of one).

What to do:

1. Copy off anything important first, if it isn’t already backed up. The rebuild that follows reads every sector of every remaining drive, and that is exactly when a second weak drive tends to fail.
2. Identify the failed drive by bay number in Storage Manager, not by guessing from the lights.
3. Replace it with a compatible drive of the same or larger capacity, then use Repair on the storage pool to rebuild onto the new drive.
4. Leave the NAS alone while it rebuilds. Large pools can take many hours or more. Avoid heavy use and don’t restart it.

Repair is the right tool here because all the other drives are healthy. It is the wrong tool in the situations below.

“Crashed” storage pool or volume

Crashed means DSM can no longer assemble the pool or mount the volume. Usually more drives have failed than the pool can tolerate, or the file system has been damaged. Shared folders may disappear, become read-only or show errors. This is a data recovery situation, not a maintenance one.

The options DSM offers at this point, such as Repair, Online Assemble, removing and re-adding drives, or reinstalling DSM, all write to the disks. If the drives are failing or the RAID information is damaged, those writes can overwrite exactly the metadata needed to put the data back together. The safest thing to do is:

• shut the NAS down cleanly if it still responds, or power it off if it doesn’t;
• label each drive with its bay number before removing anything;
• don’t put the drives into another NAS or a PC that offers to “initialise” or format them;
• write down what Storage Manager showed, or take screenshots, including any drive marked Crashed, Initialised or Not Initialised.

Drives showing “Initialised” or “Not initialised”

If a drive that was part of a working pool suddenly shows as Initialised or Not Initialised, often after a power cut or a drive being moved to a different bay, DSM no longer recognises it as a member of the pool. It is tempting to add it back to the pool, but that can start a rebuild across the wrong data. Unless you are certain which drive belongs where and that the rest are healthy, treat it like a crashed pool and stop.

Read-only Btrfs volumes

A Btrfs volume that suddenly becomes read-only usually means DSM has detected file system errors and is protecting the data from further damage. Copy off what you can while it is readable. Don’t run repeated scrubs or file system repairs on a volume that sits on failing drives: they stress the drives and may not help.

Common causes worth knowing

• Two drives from the same batch failing close together. Drives bought together, worked equally hard, often wear out together. The first failure is a warning about the others.
• Power cuts during writes, which can damage the file system or leave drives out of sync. A small UPS connected to the NAS lets it shut down safely.
• Ransomware, which encrypts files over the network. The pool stays Healthy, but the data is useless. Btrfs snapshots that can’t be changed from the network are a good defence.

When to get help

If your pool is Degraded, all other drives are healthy and you have a backup, replacing the failed drive yourself is reasonable. If the pool or volume is Crashed, more than one drive is reporting problems, or there is no backup of data you need, power the NAS down and get advice before trying anything else. Our Synology NAS data recovery page lists the models and faults we deal with, and SHR and Btrfs recovery covers how mixed-size SHR volumes are rebuilt. For other makes, see NAS data recovery.

Call 020 7064 4996 or request a quote for a free assessment. An engineer will tell you what can be recovered before any work starts.