Synology CloudStation Sync Stops Working — 200,000 File Limit

Section: Network & Storage Tags: synology, cloudstation, cloud-drive, sync, file-limit, nas, dsm Source tickets: #27681 Confidence: High — confirmed by Synology support after escalation

Symptom

Files on the Synology NAS are not syncing to one or more client machines via CloudStation Drive (now Synology Drive). Users report:

Files visible on server but not appearing on client
Folder moved on client does not reflect on server (or vice versa)
CloudStation Drive shows "syncing" indefinitely or is stuck
After a folder was accidentally moved and moved back, sync never recovered

Root Cause

Confirmed by Synology Support: CloudStation Drive (and its successor Synology Drive) supports a maximum of 200,000 files per sync task. If the file count in the synced folder exceeds this limit, the sync database becomes inconsistent. The client application may appear to work (it will attempt to sync) but silently fails to propagate changes.

Accidentally moving a large folder tree and moving it back can corrupt the local CloudStation database independently, causing files to be placed in wrong locations after a reinstall.

In the confirmed case: the server-side file count was 2,967,651 — nearly 15× the supported limit.

Prerequisites

Admin access to Synology DSM
Ability to check the file count on the sync share
Client machines with CloudStation Drive or Synology Drive installed

Diagnosis Steps

Check current file count on the NAS

Log in to DSM
Open File Station
Right-click the folder being synced → Properties
Note the file count shown

If count > 200,000: the sync tool is unsupported for this volume. Proceed to the resolution below.

Remediation Steps

Step 1 — Stop all sync tasks immediately

On each client machine: open CloudStation Drive / Synology Drive → pause or stop all sync tasks
In DSM: CloudStation Server / Synology Drive Server → Sync Tasks — stop all active tasks
Do not attempt to restart sync until the file count is resolved

Risk: None — no data is lost. Files remain in both locations.

Step 2 — Restructure the sync scope to stay under 200,000 files

The sync task must be split so no single task covers more than 200,000 files. Options: Option A — Exclude large subfolders from sync

In CloudStation Drive / Synology Drive on the client: edit the sync task → Selective Sync → untick the largest subdirectories (archive folders, old project data, media libraries)
Keep only active working folders in sync

Option B — Create multiple separate sync tasks by department/folder

In DSM: CloudStation Server → Sync Tasks → Create — create one task per major top-level folder
Each task must cover fewer than 200,000 files total
Assign each task to the relevant users only

Option C — Move archive data off the sync share

Move infrequently accessed data to a separate NAS share that is not included in any sync task
Access via mapped drive rather than sync

Expected result: Each sync task covers well under 200,000 files. Proceed to Step 3. Risk: None to data — only sync scope changes.

Step 3 — Clear the CloudStation database on each client and re-sync

After restructuring scope, the local database must be rebuilt cleanly:

Stop CloudStation Drive / Synology Drive on the client
Delete the local database folder:

- Mac: ~/Library/Application Support/CloudStation/ - Windows: C:\Users\<username>\AppData\Roaming\CloudStation\

Relaunch CloudStation Drive — it will re-index and re-sync
Allow the initial sync to complete fully before considering the task resolved (may take hours for large datasets)

Expected result: Clean sync completes without errors. Files visible on both client and server. Risk: None — the local database is a cache. No user data is in it.

Verification

After sync completes:

Create a test file on the client
Confirm it appears on the NAS within 30–60 seconds
Create a test file in DSM File Station
Confirm it appears on the client within 30–60 seconds

Prevention

When initially scoping a CloudStation / Synology Drive sync task, check the file count first (File Station → right-click → Properties)
For file shares above 150,000 files: use mapped drive access (SMB) instead of sync, or use Hyper Backup for server-to-server replication
Monitor file count quarterly as the business grows

If This Fails

If the file count is within limits and sync still fails, check DSM logs: Log Centre → Logs → CloudStation for specific error codes. Common follow-ups:

Port 6690 blocked by firewall (CloudStation's sync port)
Clock skew between client and NAS > 5 minutes (NTP issue)
DSM CloudStation Server package needs updating