Synology NAS Error Code 21: How to Fix It

Easy 30-90 minutes Medium Severity Verified June 2026
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Error Code
21
Brand
Synology
Product Type
nas
Severity
Medium
DIY Difficulty
Easy
Estimated Fix Time
30-90 minutes
Synology NAS Error Code 21 appears when a storage volume on your Synology device is running critically low on disk space, triggering a DSM alert to warn you before the drive fills up completely. If left unresolved, a full volume can cause new files to fail to save, running services to crash, and even corrupt existing data. Acting quickly to free up space or expand storage will resolve this warning and keep your NAS running smoothly.
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Tools You'll Need

How to Fix Error Code 21

  1. Log Into DSM and Check Storage Usage

    Do not ignore this warning. If the volume reaches 100% capacity, running applications and Docker containers may crash, shared folder writes will fail, and data corruption is possible.

  2. Identify the Largest Space Consumers

  3. Empty the Recycle Bin on Shared Folders

    Files deleted from the Recycle Bin are permanently removed and cannot be recovered. Double-check that you do not need any of these files before emptying.

  4. Delete or Move Unnecessary Files and Backups

    Before deleting backup files, verify you have at least one other copy of critical data stored elsewhere. Never delete your only backup of important files.

  5. Manage Snapshot and Backup Retention Policies

  6. Review and Clean Up Package Data

  7. Enable Storage Usage Notifications

  8. Expand Storage Capacity If Needed

    Always power down the NAS safely using the DSM shutdown option before physically installing or swapping hard drives, unless your model explicitly supports hot-swapping. Back up all data before beginning any drive replacement procedure.

Parts You May Need

NAS-compatible hard drive
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External USB hard drive
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NAS-compatible hard drive
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External USB hard drive
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NAS-compatible hard drive
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External USB hard drive
Check Price on Amazon
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When to Call a Professional

Contact Synology support or a qualified data storage technician if you suspect a hard drive is failing alongside the full-volume warning (look for drive health alerts in Storage Manager), if you are unable to log into DSM, if data appears corrupted or missing, or if you need help planning a storage expansion for a business-critical NAS deployment. If a drive shows 'Degraded' or 'Crashed' status, stop all write operations immediately and seek professional data recovery assistance.

Frequently Asked Questions

What does Synology Error Code 21 mean?
Synology Error Code 21 is a DSM alert indicating that one of your NAS storage volumes is nearly full. DSM triggers this warning to give you time to free up space or expand storage before the volume reaches 100% capacity, which could cause data loss or service failures.
Will Synology Error Code 21 cause me to lose data?
Error Code 21 is a warning, not a failure — your data is safe at this point. However, if you ignore it and the volume fills up completely, new file writes will fail, running services like Surveillance Station or Docker may crash, and in some cases existing data could become corrupted. Addressing it promptly prevents any data loss.
How do I quickly free up space on my Synology NAS to clear Error Code 21?
The fastest ways to free up space are: (1) Empty the Recycle Bin on all shared folders via File Station; (2) Delete old snapshots in Snapshot Replication; (3) Remove outdated Hyper Backup versions; (4) Delete completed downloads in Download Station; and (5) Remove unused Docker images. Use the Storage Analyzer package to pinpoint the largest files and folders first.
How do I stop Synology Error Code 21 from happening again?
Set up proactive storage notifications in Control Panel > Notification to alert you when volume usage exceeds a threshold like 75% or 80%. Also configure automatic retention policies in Hyper Backup and Snapshot Replication to limit how many backup versions are kept. Regularly reviewing Storage Analyzer reports is also a good habit.
Can I expand my Synology NAS storage to fix Error Code 21 permanently?
Yes. If your NAS has empty drive bays, you can add additional hard drives and expand the volume in Storage Manager. If all bays are occupied, you can replace existing drives with higher-capacity ones one at a time — DSM will rebuild the RAID and allow you to expand the volume after each replacement. Some NAS models also support Synology Expansion Units for additional bays. Always check your NAS model's compatibility list before purchasing drives.

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