Why does deleting data not free up space on an external RAID connected to macOS?

Last modified date: 2026-04-13

Applicable Products

QNAP TR Series (Direct-Attached Storage)
External RAID devices connected via USB or Thunderbolt
macOS systems using APFS file systems

Symptom

After deleting a large amount of data from an external RAID volume on macOS:

The available storage space does not increase
Finder shows significantly less data than expected
df reports high disk usage (for example: ~21 TB used)
du reports much lower actual file usage (for example: ~11 TB)
Trash has already been emptied
No large hidden folders (such as .Trashes) are found

Root Cause

This behavior occurs because APFS snapshots retain references to deleted data. When snapshots exist (such as those created by Carbon Copy Cloner or Time Machine):

Deleted files are not fully removed until snapshots are deleted
Data remains referenced by at least one snapshot
The file system does not release the occupied space

As a result:

df includes snapshot data in used space
du only accounts for active (visible) files
This creates a discrepancy in reported space usage

Important: This process is managed by macOS and the APFS file system, not by the QNAP external RAID device itself.

Background

APFS uses snapshots to preserve point-in-time states of a volume.

If a snapshot references data blocks, those blocks cannot be reclaimed—even after files are deleted.
In real-world cases, Carbon Copy Cloner (CCC) often creates snapshots (e.g., com.bombich.ccc.*) that may retain terabytes of deleted data.

Solution

To recover space, you must remove the APFS snapshots:

Step 1: Check for APFS snapshots
Run the following in Terminal:

diskutil apfs listSnapshots /Volumes/<YourDriveName>

Step 2: Identify snapshot source

com.apple.* → macOS / Time Machine
com.bombich.ccc.* → Carbon Copy Cloner

Step 3: Remove snapshots

Option A (Recommended):Use the backup software
- Open Carbon Copy Cloner or related app
- Go to snapshot management and delete old snapshots

Option B (Advanced Users):Use Terminal

diskutil apfs deleteSnapshot /Volumes/<YourDriveName> -uuid <SnapshotUUID>

Step 4: Verify space recovery
Run:

df -h /Volumes/<YourDriveName>

The available disk space should now reflect the reclaimed capacity.

Limitations

Disk Utility “First Aid” does not remove snapshots
Snapshots are not visible in Finder
External DAS devices (e.g., TR series) do not control file system behavior
Space reclamation is entirely managed by macOS