Replicate Synology NAS Data to a SharePoint Site with Cloud Sync

Many organizations use a Synology NAS as primary on-premises storage but want specific folders — project files, departmental shares, archive data — to be continuously replicated to a SharePoint Online site. This gives users cloud access, enables collaboration through the Microsoft 365 stack, and provides an off-site copy for resilience.

Synology DSM ships a free package called Cloud Sync that connects natively to Microsoft SharePoint Online. It supports one-way (upload-only), one-way (download-only), or bidirectional synchronization on a schedule or in real time. This recipe walks through configuring Cloud Sync against a SharePoint site and highlights the pitfalls (filename restrictions, path length, permissions) that commonly break the first sync attempt.

When to use this recipe

• You want NAS folders mirrored into a SharePoint document library for cloud access.
• You need an off-site replica of NAS content that fits inside your existing M365 tenant.
• You're implementing a hybrid workflow where some users edit on the NAS (SMB) and others edit in SharePoint/Teams.

When not to use this recipe

• You want to back up SharePoint to your NAS — use Active Backup for Microsoft 365 instead (opposite direction).
• You need a tamper-resistant, versioned backup with retention — use Hyper Backup targeting object storage (S3/Azure Blob), not SharePoint.

After completing this recipe you will have:

• The Cloud Sync package installed on your Synology NAS (DSM 6.1 or later).
• An authenticated connection from DSM to your Microsoft 365 tenant scoped to a specific SharePoint site.
• A configured sync task that maps a local NAS folder to a SharePoint document library with your chosen sync direction.
• Understanding of the filename, path-length, and permission limitations that affect SharePoint sync.
• A monitoring workflow — Cloud Sync logs and error reporting — so failed files are visible.

Data and compliance risks

• Permissions do not replicate. NAS ACLs (SMB/AFP permissions) are not converted to SharePoint permissions. Anyone with access to the target library can read synced content. Review SharePoint site membership before the first sync.
• Sensitive data may leave on-prem. Uploading to SharePoint moves data into Microsoft's cloud — verify this is allowed by your data residency / classification policy.
• Bidirectional sync can cause deletion loops. If a user deletes a file in SharePoint and bidirectional sync is enabled, it will also be removed from the NAS. Start with Upload local changes only unless you specifically need two-way editing.

Technical gotchas

• Filename restrictions: SharePoint rejects names containing * : < > ? / \ | and names like CON, PRN, AUX. These files will be logged as errors and skipped.
• Path length: SharePoint's full URL path limit is ~400 characters. Deeply nested NAS folders will fail partway through.
• File size: SharePoint caps single files at 250 GB. Disk images, VM files, and large archives may not sync.
• Throttling: Microsoft 365 throttles Graph/SharePoint traffic. Large initial syncs can take days and trigger temporary 429s — Cloud Sync will back off and retry.
• Token expiry: The OAuth token used by Cloud Sync can expire if the admin account's password is rotated or MFA policies change. Use a dedicated service account where possible.