What causes TimeMachine backups to fail on NAS devices?

I spoke Apple support today. I got a senior advisor who escalated it to engineering. Engineering decided that it was my problem and I should call Synology. I don't believe that it's Synology's problem based on what I've read here. But unless others call they will pretend it's someone else's fault. The senior advisor was very pleasant and helpful. He says the engineering team counts the number of problem reports and will act if enough people complain.

It might be helpful if people would contact Apple support: https://getsupport.apple.com/solutions/call. The wait time was not long when I called. If you call make sure you get to a senior advisor who can escalate. They will take some data from your machine so do this when you have some time and are near your Mac and backup server.

So last night I managed to make the backup that had eluded me. If you've read my previous posts, I have an old Synology NAS and a new one. I had failed until turning off the firewall on 15.0.1 to make a backup on the old, which caused me to buy the new. 15.1 fixed the problem on the old, but I could not get it to work on the new one. Yesterday I made a new copy of the sparsebundle from the old on the new. That one I was able to back up to.

I don't know if this is a permanent fix. The copy was almost up to date, so the backup didn't need to be connected very long. When it failed before it would work for a while and then say "can't connect", which was wrong because it had been connected. I'm going to watch it for a week or two and backup to both machines.

It's possible that some of the problem is that 15.0.1 was bad and corrupted the backups. It may be that for some making a fresh backup will work.

Good luck to all of us (and Happy Thanksgiving to those in the US)

This is what I'm trying. Downgrading back to 15.0 per support page. How to download and install macOS - Apple Support

Backing up to my Synology NAS, using Sequoia 15.0.1 (24A348), only works in Safe Mode.

Some success now!

After trying all of the above in my previous post, I was FIRST able to successfully complete a Time Machine backup to the external NAS (a WD 6TB hard drive) on the Airport Extreme. Not sure why it worked this time. But the backup to the M1 had failed again.

Finally, I tried one more setting change on the Mac Mini, in the energy saver section: I switched OFF the option to put hard disks to sleep when possible. Actually, the Mini has external SSDs, no external USB hard drives. Also, the switch for prevent sleep when display is off was switched OFF on the Mini.

This time the backup to the M1 Mini from the MacBook succeeded. Possibly noteworthy is that my MBP has about 750 GB used, and the .sparsebundle file on the Mini is just over 1 TB in size. I do have a Netgear Orbi A3000 mesh wi-fi network with 2 satellites (RBR50 and RBS50 x 2) with Backhaul enabled over which the Time Machine backups occur.

While I'm glad the backups have completed, I'm not really sure if any of the changes I made really helped.

This method not appear to work for me, unfortunately. I'm also backing up with Time Machine to Synology NAS.

Tried removing the Time Machine target in System Settings, and disabled/reenabled "Bonjour Time Machine broadcast via SMB" in DiskStation's Control Panel > File Services, re-added the Time Machine target. Still getting the error: "The network disk disconnected from your Mac while backing up."

I have two Synology NAS devices, and several Macs running MacOS Sequoia 15.0.1 – none of them can currently finish a Time Machine backup to any Synology NAS. However, all of the Macs could complete Time Machine backups to the same NAS devices in version MacOS Sequoia 15.0.

Reach out to the developer would be my suggestion. I had a problem with my VPN (VPN Unlimited) and they had a new version available via the App Store than what I was running from their website and problem was resolved.

I have the same setup on 4 x machines and it 15.1 is working fine, 3 x MacBook Pros and 1 x MacBook Air.

Little Snitch 6.1.2

I guess you could uninstall Little Snitch to check if it is that getting in the way?

Adrian.

I may have a clue about what the problem is.

I have an old Synology NAS DS213j. Under 15.01 I needed to turn off the firewall in order to make a backup. Somehow when I upgraded to 15.1 things worked. I though the bug went away.

But it scared me enough that I decided I should get a NAS that's not a decade old and can run modern software. So I bought a Synology DS224+ and installed it. I tried to backup using timemachine to it. It fails in the way everyone's been reporting. Turning off the firewall doesn't help. BTW 15.1 vs 15.1.1 doesn't make any difference.

There are two large differences between the working old machine and the new one. The new one is much faster. And the new one is running the 7 Synology OS where the old one is running the 6. I've done my best to use the same settings but in the new OS.

My guess is the bug has something to do with a timing error, and that turning off the firewall perturbs the timing enough that it doesn't appear for some people.

I'll try to call Apple tomorrow when their first string help desk is available. But I don't expect to get any more success than everyone else has had.

When my display is asleep and Time Machine attempts to run, it fails and presents the error message "Time Machine couldn't back up...". Waking up the display and manually selecting a backup results in success.

This bug has been reported for months in various other forums.

Yet another Sequoia bug. C'mon Apple, get this fixed and do better with your QC in advance of new OS releases.

After upgrading to 15.1, Time Machine to my NAS is working properly.