Timemachine Back-up in MacOS 26 (Tahoe) does not work on Synology DS920+ NAS

I am experiencing the same issue. Based on the analysis so far, it appears to be a macOS 26 problem rather than an NAS issue. When the existing backup is deleted from the NAS, and a new storage is added followed by starting the backup, the network transmission increases to around 300 MB but then stops shortly afterward. At that point, folders and files are created normally on the NAS. If it had been a permissions issue, the connection would have failed from the beginning.

It seems that the problem occurs during the process of creating a Time Machine backup on macOS.

I hope this can be resolved soon.

Hello,

I have the same problem with a new clean installation of macOS Tahoe 26. I back up data not only to Synology but also to Truenas, using the SMB protocol in both cases. Backups do not work in either Synology or TrueNAS. The backup stops at the "Preparing" stage, regardless of whether encryption, username, or password is enabled. On my other laptop with macOS Sequoia 15.7, backing up using the SMB protocol works without any problems.

In my case, the issue was caused by Korean characters included in the Time Machine image. After changing the image description to use only English, the backup proceeded normally. Thank you. karel250

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I have a Synology DS223j configured based on Synology's kbase article, and it's backing up my 16" M4 Pro MacBook Pro running macOS 26 just fine, and has been doing so since I installed Tahoe yesterday. Last backup completed a few minutes ago.

Have you tried restarting the NAS?

I have two NAS, one Synology and one QNAP, and after installing Tahoe both time machine backups stopped working. Time machine says that the backup location is unavailable after a short while. I have not touched any configuration. Deleting old backups makes no difference. Tahoe is broken

Aha, unfortunately I cannot offer any other advice, I am not an IT expert. I consulted this problem with Apple and Synology and after sending logs from the NAS I received this response from Synology. Your problem is probably somewhere else. I recommend contacting Synology – I believe you have a DS224+ as you wrote, you will have to get through AI to a live technician who will guide you on how to send logs from the NAS. Once you upload them to Synology support, within about 3 days the logs are reviewed and they will send a solution. I have had a good experience with Synology. Unfortunately, I do not have any other immediate advice… I am sorry… I am a lawyer and IT is just my user hobby xD

Thank you. Backups finally working again. Problem was Záloha (translated Backup) in image name / description or whatever it is.

Also on the previous version of MacOS I had problems using my Synology for Time Machine backups. My MacBookPro M1 would loose connection to the NAS every time it came back from sleep. I fixed this installing Automounter, since then I had no problems. Until Tahoe.....

In order to have Time Machine backups with Tahoe on my Synology I had to start from scratch...

I’m experiencing the same issue – Time Machine shows the following error: “Time Machine can’t complete the backup to ‘Time Machine’. The backup disk is not available. Make sure it is connected or select a different backup disk.”

Regular connection to the NAS disk over the network works fine. However, after the update, direct access to the NAS via the Finder sidebar (where I had created a shortcut pointing directly to a specific folder on the NAS) also stopped working, even though it used to work before. Creating a completely new backup on a new disk on my Synology NAS (DS418) did not help either.

After I updated to Tahoe, TimeMachine started to backup on the NAS On the existing AFP share, but stopped. After restarting it stopped again.

Then I deleted the existing Back-up and I configured the NAS as required with SMB.

Now I get the login errors.

The other Shared folders are only accessible when I configure AFP again.

As workaround I have my personal data files stored on iCloud-drive.

Besides my Synology DS224+ NAS unit, I also installed one Unifi UNAS2 NAS unit, but experience exactly the same result, it doesn´t work with MacOS 26 computers. Earlier MacOS computers work just fine with the Time Machine.

I tried following the instructions on the DS 120J, which I use just for storing backups, but it still doesn't work. It transfers the profile, starts up, and then immediately stops with an error saying that the disk has been disconnected. Which is nonsense because I can see it in Finder. I have an exception in my firewall and everything is OK. The update just somehow ruined the whole Time Machine.

Time machine stopped backing up to my NAS DS423, the 15/9, the same day i updated to MacOs 26. Before that date backup worked like clock. I´ve been through all loops and hoops I can think of to no avail.

I have other software like Lightroom that flawlessly backup to my NAS and software that uses the NAS for different purposes.

Whatever I do all i get is the message that ”DS423.local is not available”. I can mount the backup folder in TM without problem but after few seconds after the backup have started i get thar error message

It works! I tried it on Synology without encryption for now, Thank you very much

Look in this thread and under my username for the solution – I posted an exact guide for your problem.