What causes TimeMachine backups to fail on NAS devices?

I back up to a Synology NAS using Time Machine and it worked yesterday and not today after 15.0.1 update

"The network disk disconnected from your Mac while backing up."

Mine gets to 0MB. But with a film playing off the same NAS during backup gets to 95 MB then goes to 'skipping'

Tl;dr. This tip also worked for me. Thanks!

Waiting fix from Apple.

Details:

I have Synology NAS and use Time Machine service (all device in same local network). It's working fine for years.

After successful update to Sonoma 15.0.1 i received error message:

The backup disk image was ejected or disconnected from your Mac while backing up.

I tried restart my devices. Also I checked wifi connection. All working fine, but backup not work.

After reading comment I just turn off firewall, and backup ended successfully.

Reading the comments I tried turning the Mac firewall off and the backup is now working fine. Thank you

I don't see that as very helpful though; the solution, not the advice LOL. I already have to run backups manually because of "failed' messages all the time. So I use it once a day manually. If I have to start turning the firewall off I don't know I can be bothered. It could be I don't need the firewall on the Mac? I don't know

same here, I thought I was losing my mind, router, or network for the past week.

Turned off Firewall in OSX 15.01 and now backup is working. Will turn it on after it fully completes. What a dumpster fire.

Strange you should say that, as I've run my Time Machine backups to my NAS for going on 4 years without problems until today with Sequoia 15.0.1/

For the past year I've been running into bug after bug after on MacOS. Now this again. It's getting rather pathetic. Apple needs to up their game and start delivering what they promise and preach. This TimeMachine/firewall thing is again another day of Apple issues. I can hardly consider this 'Pro'. This stuff breaks flow.

You know what would be ballsy. If Apple would refrain from a MacOS release next year and just fix all of the bugs and refactor the OS to an extend that their primary workstation OS doesn't feel like somebody's hobbyproject abd an experiment in quality control abstinence.

If one does a deep internet search, then one finds TM fails under Mac OS Sequoia. Some hacks (like those posted here) have been shown to work but there is no general solution yet. TM fails with NAS devices but also local SSDs regardless of firewalls and other idiosyncrasies in the system. (It still happens under 15.1.1.) Some have had success with stopping and restarting firewalls for network systems. Some have had success reformatting the local SSDs with some of those people saying the speed of backing up becomes painfully slow.

This is a HUGE bug in the OS that Apple needs to address. I suspect that the error has been known for months but MacOS was pushed out the door because they were releasing iOS 18.

Each year Apple updates Mac OS and something breaks. Usually something major. The frustrating thing is these MacOS updates appear to be motivated solely by the iOS updates. Apple has trained iPhone users to expect yearly hardware and software updates. AI features of dubious value aside, are yearly updates really needed for MacOS?

Here is a link for you to voice your concerns to Apple

Feedback - Time Machine - Apple

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.

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.

Exactly the same here. Access to the NAS via Finder also not possible anymore:

Nothing has changed in my setup, except installing upgrade to 15.0.1. Since then I get this in the console:

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

15.1 with Firewall on + Little Snitch still does not working for me. It also says Drive disconnected sometimes and "in use" but it isn't.

Sequoia 15.1 does not fix the problem. It makes it worse: the workaround of turning off the Mac Firewall which worked with 15.0.1 does not work anymore.