Time Machine + Synology (Full Backups Every Time)

Synology
1

Hi everyone,

I’m having a persistent issue with Time Machine on macOS (Tahoe) with my Synology NAS, and I’d like to know if anyone else has experienced this or found a reliable fix.

Problem:

Time Machine keeps doing full backups every time instead of incremental ones.

The first full backup is expected, but several hours later when TM runs again, it starts another complete backup from scratch (hundreds of GB), as if the previous backup didn’t exist.

1To+ in one day !

My setup:

  • MacBook Pro (APFS internal disk)

  • Synology NAS using SMB for Time Machine

  • Dedicated user created for TM

  • Shared folder “Time Machine Backup”

  • SMB settings on macOS customized via nsmb.conf

  • TM sparsebundle freshly created

  • No encryption issues

  • No network drops

  • TM mounts the NAS correctly under .timemachine/

What I observed:

  • The sparsebundle mounts as a disk image (disk4) containing an APFS container (disk5) and a volume named “Sauvegardes de MacBook Pro de xxxx” — this is normal behavior for SMB TM.

  • However, the Mac sometimes creates an extra mount point under /Volumes/ with the same name.

  • Even after cleaning up all local APFS TM snapshots, deleting ghost mountpoints, ejecting the disk image, and recreating the sparsebundle, Time Machine still performs a complete backup instead of incrementals.

  • tmutil status always shows a full-size totalBytes, matching the entire disk (~350GB+).

What I’ve already tried:

  • Deleted all local Time Machine snapshots (diskutil apfs deleteSnapshot)

  • Removed stale mount points from /Volumes/

  • Fully disabled TM via tmutil disable

  • Stopped all TM processes (backupd, diskimages-helper, etc.)

  • Ejected all disk images (hdiutil detach, diskutil eject)

  • Deleted the sparsebundle from the NAS and recreated it

  • Confirmed SMB v3 signing and streams settings

Symptoms remain:

Every time Time Machine run, even after a clean backup, it treats the next run as a new destination, resulting in another full backup.

My question:

Why is Time Machine failing to recognize the previous backup on the Synology, and forcing a full backup each time?

Is this a known bug with macOS Tahoe, Synology SMB Time Machine, or something specific to APFS/sparsebundle matching?

Any insight from people using Time Machine over SMB on Synology would be greatly appreciated.

2

Although I have successfully used Time Machine with Synology for several years, I have not yet upgraded to macOS 26 Tahoe (I intentionally am a year behind, running the latest macOS 15). There are currently several issues with Tahoe and SMB.

If you have a Synology Plus model, you might want to consider leaving Time Machine and switch to Active Backup for Business.

3

Summary of the actions that fixed the issue

I was facing an issue where Time Machine performed a full backup every time, instead of incremental backups, even after completely recreating the setup on my Synology NAS (new folder, new user, new sparsebundle, etc.).

The solution was:

1. Remove Time Machine-related extended attributes from macOS

(macOS was keeping an outdated internal identity that no longer matched the new backup)

Commands used:

sudo xattr -d com.apple.backupd.BackupMachineAddress /System/Volumes/Data
sudo xattr -d com.apple.backupd.ComputerName /System/Volumes/Data
sudo xattr -d com.apple.backupd.HostUUID /System/Volumes/Data
sudo xattr -d com.apple.backupd.ModelID /System/Volumes/Data
sudo xattr -d com.apple.timemachine.private.structure.metadata /System/Volumes/Data

2. Delete all local APFS snapshots

tmutil listlocalsnapshots /
sudo tmutil deletelocalsnapshots <ID>

3. Reboot macOS

This forces Time Machine to recreate a clean internal identity.

4. Reconnect the Synology SMB share cleanly

Then re-add it as a fresh Time Machine destination.

5. Run the first full backup

→ then trigger a second backup
→ which finally appeared as incremental, as expected.

Result

Time Machine now works normally again. Incremental backups run properly.
The root cause was outdated or corrupted internal Time Machine metadata on macOS — a known issue under macOS Tahoe when using SMB network backups.

3 Likes
4

Thanks for sharing your solution. Just a question regarding the bit above: how do you know it was full backups and not incremental? Is it with the tmutil command? And if so, does that mean the problem had nothing to do with Synology? I want to check if the same’s happening to me.

1 Like
5

In my case, it was quite easy to confirm that Time Machine was doing full backups instead of incremental ones, simply by looking at:

1. The amount of data transferred each time

Every backup was transferring around 300–400 GB, which is the full size of my internal disk.
An incremental backup should normally only transfer a few MB or a couple of GB, depending on what has changed.

So when you see Time Machine pushing hundreds of gigabytes every time, that’s a clear sign it is not doing incrementals.

2. The time required for each backup

Each backup was taking 3 to 4 hours, exactly like the very first initial backup.
A normal incremental run usually takes seconds to a few minutes.

So even without using tmutil, the behaviour was obvious:

  • Long backup time

  • Huge volume transferred

  • Fans spinning

  • Disk activity constantly high

If you experience the same symptoms — long duration + huge amount of data — then your Mac is most likely doing full backups too.

1 Like
6

Thanks for the fix…. It worked, but then 3 later the issue has returned. Every backup is a full one

Any ideas?

7

Has there perhaps been a macOS update in the meantime?

8

Thanks for the suggestion, but sadly, no. Tahoe 26.2 was installed on 14th December, which is probably when the issues began.

1 Like
9

Although I have successfully used Time Machine with Synology for several years, I have not yet upgraded to macOS 26 Tahoe (I intentionally am a year behind, running the latest macOS 15). There are currently several issues with Tahoe and SMB.

If you have a Synology Plus model, you might want to consider leaving Time Machine and switch to Active Backup for Business.