QNAP NAS Error Code 21: How to Fix It

Easy 30-90 minutes (varies based on data size and chosen solution) Medium Severity Verified June 2026
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Error Code
21
Brand
QNAP
Product Type
nas
Severity
Medium
DIY Difficulty
Easy
Estimated Fix Time
30-90 minutes (varies based on data size and chosen solution)
QNAP Error Code 21 is a volume warning that appears when your NAS storage volume is running critically low on free space. This warning is triggered to alert you before the volume fills up completely, which could cause write failures, data corruption, or service interruptions. Acting quickly to free up space or expand storage capacity will resolve the warning and keep your NAS running smoothly.
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Tools You'll Need

How to Fix Error Code 21

  1. Check Current Volume Usage in QTS

  2. Delete Unnecessary Files and Folders

    Back up any important files before deleting them. Deletion from the recycle bin is permanent and cannot be undone.

  3. Remove Old Snapshots

    Deleting snapshots removes your ability to restore data from those points in time. Confirm you no longer need the snapshot data before deleting.

  4. Clear Application Logs and Temporary Data

  5. Move Data to an External Drive or Cloud

    Ensure the external drive is formatted in a compatible file system (exFAT, NTFS, or EXT4) before transferring critical data. Verify transfers completed successfully before removing the source files.

  6. Expand the Volume by Adding a New Drive

    Always power off the NAS before physically installing a drive unless your model explicitly supports hot-swapping. Use drives on QNAP's official hardware compatibility list to avoid errors.

  7. Replace an Existing Drive with a Larger One

    Do not remove more than one drive at a time from a RAID array unless instructed by QTS. Removing multiple drives simultaneously can cause catastrophic data loss. Ensure you have a current backup before beginning this process.

  8. Adjust the Volume Warning Threshold

    Raising the threshold too high may cause you to miss critical low-space warnings in the future. Only adjust this setting if you have a solid storage management plan in place.

Parts You May Need

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3.5-inch NAS hard drive
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2.5-inch NAS SSD or hard drive
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When to Call a Professional

Contact QNAP support or a qualified NAS technician if: your volume shows errors or corruption alongside the space warning; RAID rebuilds fail repeatedly after adding a new drive; QTS reports drive failures or bad sectors during the expansion process; you are managing a business-critical NAS with important data and are not comfortable performing storage pool expansions yourself; or if the error persists after following all steps and you suspect a firmware or software bug. QNAP's official support portal at support.qnap.com offers live chat and ticket-based support for complex storage issues.

Frequently Asked Questions

What percentage of storage triggers QNAP Error Code 21?
By default, QNAP triggers the volume nearly full warning (Error Code 21) when storage usage reaches approximately 80-85% of the volume's total capacity. The exact threshold can be customized in QTS under 'Storage & Snapshots' > 'Global Settings' > 'Notification Settings.'
Will QNAP Error Code 21 cause data loss?
Error Code 21 itself is a warning and does not cause data loss directly. However, if you ignore the warning and the volume fills up completely (100%), new write operations will fail, which can lead to file corruption, application crashes, or service interruptions. It is important to act on this warning promptly.
How do I find out what is taking up the most space on my QNAP NAS?
Log in to QTS and open File Station. Click on the shared folder you want to investigate, then sort files by size (largest first) to quickly identify the biggest space consumers. You can also use the 'Storage Analyzer' app available in the QNAP App Center, which generates detailed visual reports of space usage across your entire NAS.
Can I add more storage to my QNAP NAS without losing existing data?
Yes. QNAP supports online volume expansion, which allows you to add a new drive to an existing storage pool or replace existing drives with larger ones without losing data. This is managed through the 'Storage & Snapshots' section of QTS. Always ensure you have a current backup before making any changes to your storage configuration, as unexpected power loss or drive failure during the process could result in data loss.
How do I stop QNAP Error Code 21 from appearing in email notifications?
You can adjust or disable the storage threshold notification in QTS by going to 'Storage & Snapshots' > 'Global Settings' > 'Notification Settings' and changing the volume usage alert threshold. You can also manage notification recipients and channels under 'Control Panel' > 'Notification Center.' However, it is strongly recommended to resolve the underlying storage space issue rather than simply disabling the alert.

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