My phone buzzed at 6:40 on a Tuesday morning with an email from a friend's Synology DS920+. Subject line: "Volume 1 has entered degraded mode." He called me thirty seconds later, already talking about pulling drives and rebuilding from scratch. That instinct, the urge to start yanking things, is exactly how a recoverable situation turns into a real data loss event.
A degraded RAID volume is not a crash. It means the array lost one of its redundant drives and is now running without a safety net, but your files are still there and still readable.
The whole game at this point is patience and order of operations.
What "degraded" actually means
RAID levels like RAID 1, RAID 5, RAID 6, and Synology's SHR keep an extra copy of your data, or parity that can rebuild it, spread across multiple disks. When one disk dies or drops offline, the array keeps serving files from the remaining drives. That state is called degraded.
The catch is that your redundancy is now spent. On a RAID 5 or a single-drive-tolerant SHR volume, a second drive failure during this window means the volume is gone and no rebuild will save it. RAID 6 buys you one more failure of headroom, which is a big part of why I push people toward it on larger arrays.
So the clock is quietly ticking, but it is not an emergency you fix in ninety seconds. If you want the underlying concept, the standard RAID levels page on Wikipedia lays out exactly how much failure each level tolerates.
Step 1: Stop touching hardware and read the status
Log into your NAS web interface, not the physical box. On a Synology, open Storage Manager and look at the Storage Pool and the HDD/SSD panel. On a QNAP, it is Storage & Snapshots. On TrueNAS, check the Storage dashboard and the Pool Status.
You are looking for two things: which specific drive failed, and whether the volume still shows as accessible. The interface will usually name the bay, something like "Drive 3" or "da2", and flag it with a red or yellow icon.
Write that bay number down. The first mistake I see constantly is someone pulling the wrong drive because they counted bays from the wrong side and killed a healthy disk instead of the dead one.
Step 2: Confirm your backup before anything else
This is the step people want to skip, and it is the one that matters most. A rebuild puts heavy, sustained read load on every remaining drive, and drives of the same age and batch often fail close together. That read stress is a common moment for a second failure.
So before you rebuild, make sure you can survive losing the whole volume.
- If you already have a working backup (external drive, cloud, a second NAS), verify it actually completed recently and that you can open a few files from it.
- If you do not have one, and the volume is still readable, copy your most important data off now, before you rebuild. An external USB drive and an evening of copying is cheap insurance.
- Prioritize the irreplaceable stuff first: photos, documents, anything you cannot re-download.
Backblaze publishes real-world drive failure data every year, and their drive stats reports are a sobering reminder that same-batch drives cluster their failures.
Step 3: Identify why the drive dropped
Sometimes a drive is not actually dead. Loose SATA connections, a drive that briefly overheated, or a firmware hiccup can knock a disk out of the array even though the hardware is fine. Check the drive's SMART status in the same storage panel.
If SMART shows reallocated sector counts climbing, pending sectors, or the drive simply is not detected at all, treat it as failed and replace it. If SMART looks clean and the drive just "disappeared," it is worth a closer look before you spend money.
Here is a rough decision guide I use.
| What you see | Likely cause | Action |
|---|---|---|
| Drive not detected, no SMART data | Dead drive or dead connection | Reseat cable, then replace if still gone |
| SMART: reallocated or pending sectors rising | Failing platters | Replace the drive |
| SMART healthy, drive dropped once | Cable, heat, or power glitch | Reseat, monitor, do not rush to buy |
| Multiple drives flagged at once | Controller, backplane, or PSU | Stop, do not rebuild, seek help |
That last row is important. If two drives failed simultaneously, the problem is often not the drives at all, and rebuilding onto a bad controller can corrupt more data.
Step 4: Get the right replacement drive
Buy a replacement that is the same capacity or larger than the failed drive. Smaller will not work; the array needs at least the original size to rebuild. Match it to a NAS-rated model where you can, something like a WD Red Plus or a Seagate IronWolf, because desktop drives can drop out of arrays under sustained load.
A 4TB NAS drive runs roughly 90 to 120 US dollars as of mid 2026, and it is not the place to save fifteen dollars on a shucked desktop disk.
Step 5: Replace and rebuild
With a verified backup in hand, remove the failed drive from the bay you noted in Step 1. Slide the new drive in fully until it seats. The system usually detects it within a few seconds.
Then trigger the repair from software, not by hoping it starts on its own.
- Synology: Storage Manager, select the degraded Storage Pool, click Repair, and pick the new drive.
- QNAP: Storage & Snapshots, select the pool or RAID group, choose Manage, then Rebuild or Replace.
- TrueNAS: Storage, open the degraded pool, find the offline disk, and choose Replace, then select the new device.
The rebuild (Synology calls it a repair, ZFS calls it a resilver) will run in the background. Expect it to take anywhere from four hours to well over a full day depending on drive size and how full the volume is. An 8TB drive on a busy array pushing 20TB of data can genuinely take 24 hours or more.
Leave it alone while it works. You can keep using the NAS, but avoid heavy transfers or big deletes until it finishes.
Step 6: Verify and harden
When the status flips back to Healthy or Normal, do not just close the tab and forget it. Run a manual SMART extended test on the new drive and, if your platform supports it, kick off a data scrub or RAID scrubbing pass to confirm the rebuild is genuinely clean.
Then fix the thing that made this scary in the first place. Set up a real backup job if you did not have one, enable email or push notifications for storage events, and if you are on RAID 5 with drives larger than about 6TB, seriously consider moving to RAID 6 or SHR-2 next time you expand.
The morning after
My friend's DS920+ finished its repair around 9pm that same day, and Volume 1 went back to green. Nothing was lost, mostly because we spent the first hour copying his photo library to a spare drive instead of ripping out hardware.
A degraded volume is your NAS doing its job: it is telling you a drive died and giving you a window to fix it calmly. Read the status, protect your data, replace the right drive, rebuild, then verify. Handle it in that order and a scary email becomes a boring maintenance task.
Frequently asked questions
Can I still use my NAS while it is degraded?
Yes. A degraded volume stays online and your files remain accessible, so you can read and even write to it. Just avoid heavy transfers or large deletions until you have a backup and have started the rebuild, since you are running without redundancy.
How long does a RAID rebuild take?
It depends on drive size and how full the volume is. A small, mostly empty array might rebuild in a few hours, while an 8TB or larger drive on a full volume can take 24 hours or more. Let it run in the background and avoid heavy activity during that time.
Do I have to buy the exact same drive model?
No, but the replacement must be the same capacity or larger, or the rebuild will not proceed. Stick to NAS-rated drives like WD Red Plus or Seagate IronWolf, since desktop drives can drop out of arrays under the sustained load a rebuild creates.
What if a second drive fails during the rebuild?
On RAID 5, RAID 1, or single-fault SHR, a second failure during rebuild usually means the volume is lost. This is exactly why you verify a backup first. RAID 6 and SHR-2 tolerate a second failure, which is why they are worth it on larger arrays.
The drive shows healthy SMART but dropped out of the array. Do I replace it?
Not necessarily. A healthy drive that dropped once is often a cable, heat, or power glitch. Reseat the connection, re-add the drive, and watch it. If it drops again or SMART starts showing reallocated or pending sectors, replace it.
Two drives failed at the same time. What now?
Simultaneous failures usually point at the controller, backplane, or power supply rather than the drives themselves. Do not start a rebuild onto suspect hardware, because that can corrupt more data. Power down, check connections and power, and get expert help before proceeding.
