Few data storage issues induce more anxiety than seeing your RAID 5 array go into a degraded state—especially when working with critical data and seemingly reliable systems like Synology NAS devices. Recently, I faced a real-world disaster: a failed parity rebuild operation that cascaded into unusable volumes and nearly complete data loss. Despite the stress, I was eventually able to recover key information and restore the storage system back to a stable state.
TL;DR: A parity rebuild on my Synology RAID 5 array failed midway due to a second disk showing abnormal SMART values. This led to multiple degraded volumes and inaccessible shared folders. By isolating the healthy drives, performing controlled power cycles, and using Synology’s command-line utilities, I was able to mount the damaged volume, backup important data, and reconstruct my RAID. Anyone facing a similar crisis can recover valuable data with methodical troubleshooting and some technical know-how.
What Went Wrong: The Initial Trigger
It all started with a standard alert from DSM (DiskStation Manager)—one of the hard drives in my five-disk RAID 5 array had failed. This wasn’t unexpected; the drive had over 40,000 power-on hours and its SMART data had shown a few reallocated sectors for months.
I replaced the defective drive with a new one of identical capacity and initiated the rebuild. Things seemed to be progressing normally for the first 12 hours, but midway through the rebuild process, DSM flagged a second drive in the array with “Bad Sector Count Warnings” and prompted urgent action. Moments later, the volume status switched to Degraded (Inaccessible).
This was catastrophic—RAID 5 arrays can only tolerate the failure of a single drive. A second underperforming drive mid-rebuild is, effectively, a nail in the coffin for parity accuracy and overall data availability.
Recognizing the Symptoms
Once DSM reported an inaccessible volume, I observed the following symptoms:
- Volume 1 status: “Degraded” and “Unmounted.”
- Access via SMB, AFP, and NFS failed across all shared folders.
- The new drive appeared healthy, but rebuilding had stopped at 63%.
- File Station refused to open, stating “Volume not available.”
Synology Support initially suggested switching out the second ‘suspect’ drive, but that plan posed too high a risk: any further disturbance to the array without an available backup could eliminate my window for recovery. I decided to attempt a rescue operation using SSH and the command-line interface.
Step-by-Step Rescue Process
1. Verify Drive Health
I SSH’d into the NAS and ran:
smartctl -a /dev/sdX
(*Replace “X” with the actual drive letter, e.g., sda, sdb*)
Only one drive showed critical warnings. This helped me determine which one likely caused the rebuild to fail. The new drive, despite being fresh, also had a higher than expected temperature—possibly indicating instability during intensive I/O operations.
2. Backup /etc/mdadm.conf and Check RAID Status
I backed up the RAID configuration file and ran:
cat /proc/mdstat
This showed that the RAID was degraded but still semi-assembled. At this point, I confirmed which disks were active participants in the array and which one had dropped off.
3. Assemble RAID Manually
After identifying healthy disks, I ran:
mdadm --assemble --force /dev/md3 /dev/sda /dev/sdb /dev/sdd
This command successfully forced RAID assembly in degraded mode. /dev/md3 now appeared with an “active (auto-read-only)” status.
4. Mount the Volume Manually
I created a mount point and attempted a manual mount:
mkdir /volume_temp
mount /dev/vg1000/lv /volume_temp
This succeeded. Although read-only, I was able to access critical files and copy backups to an external USB 3.0 drive.
5. Store Recoverable Data
I mounted a USB drive and began a full Rsync pull:
rsync -avh --progress /volume_temp/usbshare/backups /usbdrive/backups
This took several hours but enabled me to secure everything essential—about 3.6TB of office documentation and media resources.
6. Remove Failing Drive & Rebuild
Post-backup, I shut down the NAS and physically removed the failing drive. Once only the three healthiest drives remained, I restarted the unit and allowed DSM to recognize the degraded array.
I initiated a new RAID rebuild using only known-good disks and replaced the removed unit with a warranty replacement. The rebuild process completed after 14 hours. The rebuilt volume was successfully remounted by DSM and transitioned back to a “Healthy” state.
Lessons Learned & Recommendations
Synology RAID arrays are usually resilient, but rebuilds are their Achilles’ heel if more than one disk has underlying issues. Here’s what I learned and recommend to others running similar setups:
- Never ignore SMART warnings. Even a single reallocated sector is a warning sign in a RAID environment.
- Schedule regular tests using DSM’s built-in utilities. A monthly health report can catch issues early.
- Keep a ‘clean’ full backup of your data, especially before initiating a rebuild.
- Use identical disk models where possible. Variations in temperature tolerance and controller timings can affect stability during rebuilds.
- Familiarize yourself with mdadm, LVM, and the DSM CLI tools before disaster strikes.
Ongoing Monitoring After Recovery
Since the incident, I’ve installed Synology’s Active Insight utility to offload health data analytics and receive predictive alerts. I’ve also replaced spin-up heavy hard drives with NAS-optimized models and changed my RAID 5 array to RAID 6 for extra redundancy.
DSM now runs scheduled smart tests weekly and emails reports directly to a dedicated mailbox. These monitoring improvements—while long overdue—can make the difference between minor disruptions and critical failures.
Final Thoughts
A failed parity rebuild on RAID 5 without proper preparation is a nightmare scenario. But it’s also a sharp reminder of the necessity for vigilance, proactive maintenance, and always having at least one solid off-device backup.
Although this experience was harrowing, it offered valuable hands-on insight into how software RAID operations work within Synology’s ecosystem—and proved that with enough planning and technical effort, data loss is preventable even in worst-case scenarios.