WD MyCloud PR4100 shows full (2x4tb) when the data is only 2.1TB

If your WD MyCloud PR4100 is showing as full (8TB total, 2x4TB drives), even though you've only stored about 2.1TB of data, it can be confusing and concerning. There are several possible explanations for this issue, often tied to the RAID configuration, system files, snapshots, or background processes.

1. RAID Configuration

One of the most common causes for unexpected storage usage is the RAID setup. The PR4100 supports several RAID modes (RAID 0, 1, 5, 10, JBOD, and spanning). If your drives are configured in RAID 1, then each 4TB drive is a mirror of the other. This means that although you have 8TB of physical space, only 4TB is usable. So, 2.1TB used out of 4TB (not 8TB) would appear as over 50% full. To verify this:

Go to the MyCloud Dashboard.

Check Storage > RAID to confirm the RAID mode.

If it’s RAID 1, the mirroring is why you see less usable space.

2. Snapshots or Backups

WD MyCloud OS supports snapshot backups, which can take up large amounts of space. If snapshots are enabled and scheduled frequently, older snapshots could be consuming hidden space. These are often not immediately visible through normal file access but are counted in total disk usage. To check:

Navigate to the Backup or Snapshot section of the dashboard.

Review and delete old or unnecessary snapshots.

3. Recycle Bin and Deleted Files

The MyCloud OS often has a Recycle Bin feature for each share. If enabled, deleted files aren’t permanently removed and still occupy space.

Go to Shares > Recycle Bin settings.

Manually empty the recycle bin for each share.

4. System and Log Files

The operating system, apps, logs, and cache also consume space. Although these shouldn't account for several terabytes, if something is malfunctioning (like excessive logging), it might eat up drive space.

SSH into the PR4100 if you're comfortable with command-line operations.

Check disk usage using df -h or inspect logs with du -sh /var/log.

5. Drive Health and Bad Sectors

A failing drive or filesystem corruption might result in improper disk usage reporting. This could lead to storage misreadings or lost space.

Run the Disk Test from the dashboard under Utilities.

Check SMART status or consider removing the drives and testing them externally.

6. iSCSI Volumes

If you've configured iSCSI targets, those volumes can reserve space even if they’re mostly empty. Reserved iSCSI space is not released until deleted.

Check under iSCSI Settings if any targets are consuming reserved storage.

7. Volume Expansion or Migration Issues

If you recently changed RAID levels or expanded storage, the system may have created duplicate metadata or temporary files that weren't cleared properly. Restarting or rechecking the volume may help.

Backup your data.

Reboot the NAS and rerun disk scanning tools from the dashboard.

Conclusion

To resolve the issue:

Confirm RAID mode and understand real usable capacity.

Check for hidden files, snapshots, or recycle bin content.

Review system logs and health diagnostics.

Clean up unnecessary files or consider reinitializing the NAS if corruption is suspected.

If after these checks the issue persists, you may want to contact WD support or consider reformatting the drives after backing up your data.