Time Machine can back up a Mac to a Synology shared folder over SMB once DSM has a writable share, SMB enabled, and the Time Machine checkbox turned on. The Mac then picks that network disk the same way it would pick a USB drive, and DSM stores a sparsebundle (or backup bundle) on the volume.

This is a local network backup path, not a full disaster plan. Pair it with the NAS overview at /what-is-a-nas-network-storage-explained and the broader layout at /3-2-1-backup-strategy-nas. Remote access patterns such as Tailscale live at /set-up-tailscale-on-a-nas. Power protection for the box itself is covered in /ups-auto-shutdown-synology-qnap-nut.

A quiet 2-bay host such as the Synology DiskStation DS225+ is a common Time Machine destination once it has NAS-rated disks and a stable LAN path.

What you need

  • A Synology NAS on DSM with at least one volume that has free space for backups.
  • SMB file service enabled (AFP is legacy; do not rely on it for modern macOS).
  • A Mac on the same LAN (or on a trusted remote path such as Tailscale) with Time Machine available in System Settings.
  • NAS HDDs sized for the Mac's used space plus growth. A CMR NAS drive such as the WD Red Plus 4TB or the Seagate IronWolf 4TB is a typical bay fill for a single Mac plus room to grow.

SMB basics for shares and authentication also sit at /wiki/smb if you want the protocol side without DSM menus.

1. Create a dedicated shared folder (or reuse one)

  1. In DSM, open Control Panel > Shared Folder, then Create.
  2. Name it clearly (for example TimeMachine or Backups-Mac). Prefer a dedicated folder so photo libraries and Time Machine bundles do not share one messy tree.
  3. Place it on a volume with enough free space. Time Machine needs roughly 2x to 3x the Mac's used disk space for comfortable local history; more is better if multiple Macs share the destination.
  4. Skip unnecessary recycle-bin or indexing extras on a pure backup share unless you have a reason to keep them.
  5. Finish the wizard, then open the folder's Edit > Advanced (or Time Machine related checkbox, depending on DSM generation) and enable Enable this shared folder for Time Machine backups when that option appears on the share itself. Some DSM builds expose the Time Machine flag under File Services instead; the next section covers the SMB side.

2. Enable SMB and Time Machine on the share

  1. Open Control Panel > File Services.
  2. Enable the SMB service. Leave minimum SMB to a modern dialect (SMB2/SMB3 era). Do not fall back to AFP for new Macs.
  3. Confirm the shared folder is listed for Time Machine (share checkbox, or the File Services Time Machine picker, depending on DSM).
  4. Apply. From another device, confirm the NAS hostname or IP answers on SMB (port 445) on the LAN.

If older macOS clients still hunt for AFP, migrate them to SMB. AFP is retired for current Time Machine network targets on Synology.

3. Create a DSM user with write access

  1. Open Control Panel > User & Group, then create a user such as tm-backup (not your daily admin login).
  2. Grant that user Read/Write on the Time Machine shared folder only. Deny unrelated shares.
  3. Set a strong password. Store it in the Mac keychain via the connect prompt later; do not embed it in scripts on untrusted machines.
  4. Optional but smart: set a user or shared-folder quota so one Mac's sparsebundle cannot consume the entire volume.

4. Connect the Mac to the share

  1. On the Mac, open Finder. Use Go > Connect to Server (or browse Network) and connect to smb://nas-hostname/TimeMachine (use your real hostname or LAN IP and share name).
  2. Authenticate as the DSM Time Machine user. Check Remember this password in my keychain if this Mac should mount automatically for backups.
  3. Confirm the share mounts and that you can create a test file, then delete it. Permission failures here become cryptic Time Machine errors later.
  4. Leave the share mounted for the first Time Machine selection, or let macOS remount it when Time Machine runs (keychain helps).

5. Choose the Time Machine disk

  1. Open System Settings > General > Time Machine (wording varies slightly by macOS version).
  2. Add a backup disk and select the Synology share (it should appear as a network volume once mounted and Time Machine-enabled on DSM).
  3. Start the first backup. Expect it to take a long time on Wi-Fi; prefer Ethernet for the initial pass when you can.
  4. After the first full backup finishes, confirm a sparsebundle (or backup bundle) appeared on the share in Finder or File Station.

Sparsebundle quirks and quotas

Time Machine on a network share typically creates a sparsebundle disk image that grows as backups accumulate. That is normal.

  • Do not casually copy, rename, or open the sparsebundle from another OS while a backup is running.
  • If a Mac is replaced, prefer migrating with Migration Assistant or attaching the old backup as a browseable history rather than hand-editing bundle files.
  • Use DSM quotas (per user or per shared folder) so a runaway Mac cannot fill the volume that also holds photos or Docker data.
  • For multiple Macs, either give each Mac its own share (cleanest quotas) or one share with enough headroom and distinct computer names so bundles do not collide.

Multiple Macs on one NAS

Each Mac should authenticate with a DSM account that can write the destination. You can share one tm-backup user across household Macs, or create one user per Mac for cleaner auditing and quotas. Keep hostnames unique. Stagger large first backups so overnight jobs do not saturate a single 1GbE link.

What Time Machine does not replace

Time Machine on a Synology is still one local copy on hardware that can flood, fail, or get ransomed with the rest of the house. It does not satisfy offsite by itself. Keep the 3-2-1 pattern from /3-2-1-backup-strategy-nas: another copy on a different medium, and one copy offsite or offline. Hyper Backup, cloud sync of critical folders, or a rotated USB disk still matter for fires and theft.

Common failures

Bottom line

Create a dedicated Synology share, enable SMB and the Time Machine flag, give a non-admin DSM user write access, mount the share on the Mac, and select it in Time Machine. Cap capacity with quotas, plan for sparsebundles, and remember this is local history, not a full 3-2-1 plan. For remote reachability of the same NAS, see /set-up-tailscale-on-a-nas. For UPS-backed shutdown so overnight backups are not interrupted by dirty power cuts, see /ups-auto-shutdown-synology-qnap-nut.