What causes TimeMachine backups to fail on NAS devices?

To be clear, it is not the apple OS firewall that was the problem for me. it was the external firewall (mcAfee, Sophos, etc). For me, I updated my Sophos firewall to the latest version and the problem went away. Sophos had a blurb about this on their support site. try updating your external firewall (not the definitions but the actual program) and see if that fixes it for you. I am now using 15.0.1 TimeMachine with no issues. Have done 5 backups perfectly.

I was having the same problems – TM backups to my Synology NAS failing midway (via samba and afps). I did as people suggested and disabled Lulu and macOS built-in firewall and the backup functionality did work, but I kept testing. I found that after permitting smbd AND Lulu as a process/application in the firewall settings, I was able then able to enable the firewall AND Lulu AND successfully perform a TM backup. I'll see how it stands over time, but I wanted to share in case others would like to try this solution and report back if it works for them, too.

On a separate issue, TM backups tend to fail if you lock the screen. Presumably, Apple's security autocrats decided this was dereliction of duty. The commandants insist we remain handcuffed to us computers during backups. Of course, anyone with an iota of understanding of human nature knows that people will do what is easiest – leave their computers unlocked and insecure. Self-defeating!

I resolved the issue after working with an Apple support person. I have an MBP M1 Pro with MacOS 15.1.1. and I had failures with TM both on NAS and hard wired backups. My workaround, as per the boards, was to to turn off the MacOS Firewall to let the backups work to completion.

However, I also have a cyber security software which includes a firewall of its own. There is a conflict between its firewall and the MacOS one starting with Sequoia 15.x. After I turned off the security software firewall and left the Mac OS one active, all TM backups work without error. The 3rd party cyber security software firewalls need to be updated for Sequoia.

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.

I have made clear that it is not just Synology. My NAS is a My Cloud EX2 Ultra by WD also fails TM backups when the MacOS Firewall is on. More importantly, it also fails with a Seagate SDD attached directly to the MacBook Pro M1 I have. Turning off the Firewall causes BOTH to start working.

[Edited by Moderator]

So for the second day, after getting TM to work with the FW off, I tried to backup without turning it off. And it failed. I even added the TM app to the rules of FW to accept incoming connections. But that didn't work either, it said the external disc had disconnected. So I turned off FW off again just for TM and it worked perfectly. This is an error introduced by Apple in 15 or 15.0.1. I will have to turn on and off FW just to get TM to work - how annoying.

Well, here's the deal:

15.0.1 made some changes and it relies on your firewall to protect you - your provider should have updated the files after the release of 15.0.1. After spending much time with a smart guy at Apple Support, we solved the problem by updating my firewall software (Sophos) and, lo and behold, the backup now works. For me, it was that simple (the hard part was getting it figured out!) Give it a try and holler back...

I'm on 15.0.1, too, but the TM backup was not working correctly. I turned off the firewall, but the problem persisted. Then, checking, I uninstalled my KTS antivirus completely because this software adds a config in the firewall; I think for it to work correctly and filter some network traffic, this and a restart, the backup started working up.

summary: 15.1 fixes it for me.

15.0.0 and 15.0.1 broke Time Machine backups for me as described in this thread. disabling Apple firewall in network settings was the workaround.

15.1 fixes it. I no longer need to disable the Apple firewall for Time Machine backups to complete on my Symplogy NAS.

Software: Little Snitch Mini and Cloudflare Warp.

hardware: MacBook Air M3 and Synology DS220+ NAS.

What has solved it for me:

Without that it is one or some or all of the below necessarily the root cause of the problem:

(1) update MacOS to 15.1.1

(2) update of Synology to DSM 7.2.2-72806 - Update 1 (meanwhile it is Update 2)

(3) removal and new configuration of Synology as Time Machine in settings of MacOS.

(4) removal of the sparse bundle from the Time Machine folder (you might delete or move the sparse bundle elsewhere) and start a fresh Time Machine Backup.

Note for the DSM Update of Synology: It may be worth checking Synology download page, whether an update could be applied manually. In my case the update was postponed due to a feature I had installed but no longer available with a newer DSM: video station.

It would be nice to know the root cause, but important is for me that the Time Machine Backup works again.

SOLVED:

What worked for me (Sequoia 15.1.1) backing up to a Synology NAS was to forget the backup destination (NAS) in Time Machine, then DELETE the backup on the NAS, then get Time Machine to create a new backup, which it completed some 19 hours later, and has been consistently backing up since.

Only thing I noticed was that when creating the new backup it asked for an encryption password - I can't recall if that was the case when I set up the original TM backup on the NAS as it was years ago. So perhaps that was the issue triggered by Sequoia 15.x? Anyway, it's working now, but I had to sacrifice the existing backups to make it work.

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.

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?

To be clear, this is a problem with non-Apple firewalls, introduced by new code in 15.0.1. 

It’s not specific to a particular target system for a time machine network backup. 

It doesn’t matter what your target system is, or what your third-party firewall is. If you put 15.0.1 onto a machine that backs up to a target elsewhere on your network, then there is a very high likelihood that it will fail. 

If you turn off the firewall (or open specific ports) there is a very high likelihood that it will work again. 

There is a very high likelihood* that this has been caused by inadequate pre-release testing of 15.0.1. 

If you want to make the strength of your feelings known to Apple, do it here:

Feedback - macOS - Apple

*¿am I repeating myself?