What causes TimeMachine backups to fail on NAS devices?

Even though AFP is not listed as a way to connect to a NAS drive for Time Machine, I did exactly that--turned on AFP on my Synology DS218J NAS, connected to the server via AFP, and *I think* I selected the AFP-connected drive designated in the Synology settings for the Time Machine backups, and so far I'm 10%/10 GB into the initial backup with the firewall turned on.

I'll have to do some more experimentation, but connecting via AFP may be a workaround.

It’s not NAS specific. It’s actually a network communications problem when the internal or even third party firewall is enabled. For me it’s the MacOS internal firewall but I’ve been reading others have a problem with third party.

Disabling the firewall worked for me. I think I recall it working once or twice without deactivating the firewall on 15.0.1.

Now after updating to 15.1, it seems to work reliably.

This is also backup to a Synology NAS.

So I can back up with Time Machine now I am on Os 15.1. But only triggering the back up manually. If it's on the normal hourly auto. setting, it looks fine but then reverts to the "server disconnected during the procedure" type error log. But at least I can now back up. Definitely not moving from Os14 for other devices.

When this thread got started, I also noticed in the popular press that Synology NAS devices were vulnerable to remote attackers. See the latest update: https://www.synology.com/en-global/security/advisory/Synology_SA_24_27 . I don't use Synology NAS, but just making the observation.

In my case, I'm doing backups in my home wi-fi LAN, and I'm doing backups from one Apple device (MacBook Pro (Mac15,7) [Source is Mac SSD internal] to 2 different destinations: 1) WD hard drive attached to an old Airport Extreme via USB-A, and 2) Mac Mini (Mac16,10) with a Samsung T7 SSD connected via USB-C. My source drive contains about 770 GB of files to be backed up via wi-fi network.

Like many others in this thread, my Time Machine (TM) backups started failing after upgrading to MacOS 15.0. The reasons for failure were sometimes due to a network disconnect by my MBP or to the remote destination device not cooperating with TM.

I've had problems of TM backups failing after OS upgrades a number of times in the past 12 years with Apple products. Generally speaking, I've had the best success by creating a new backup (after first moving the old backup destination files to a different device, so I could restore from it if necessary). The new backup can take days to complete, so I do what is needed to speed that up (since failures would often happen after many hours had expired).

After the first full backup was successfully completed, then I changed my Lock Screen settings back to what I normally prefer (on both source and destination).

Most recently I changed Lock Screen settings while keeping the MBP connected to power with lid open, so it wouldn't go to sleep during the TM backup. Likewise for the destination machine (Mini) and its external drive. I manually started the backup, which took about 22 hours for about 750 GB, not letting either the source or destination sleep.

Also, it seems that, over the years, MacOS has changed some of the instructions for TM, depending on the OS version, especially in how to prepare the destination. I have found it worthwhile to read the Apple Support docs for the current OS version, and checking all of the various settings (e.g., especially for file sharing on the destination and settings for folders for TM backups).

Sometimes my antivirus software has interfered (BitDefender) and I have set my Time Machine protection to Off.

I have not changed MacOS firewall settings on either source or destination. I keep the firewalls on.

Yeah, this is annoying and takes time to debug. There's lots of places where problems can occur, and that's what makes it so perplexing and frustrating at times. I also use other backup strategies besides TM for really important files, using external cloud services for those.

I would try it with the Firewall turned off if you haven't already tried it.

Fixed in 15.1

I've had the same problem too. Time Machine backups started to fail after upgrade to OS 15.0.1. Both on a network attached storage (NAS) device AND on a Mac Mini as backup destination.

On the Mac to be backed up (MacBook Pro M3, source), I went to Settings, then Time Machine, and removed the destination that was failing. But I could not add it again until I did the following.

Backup disks you can use with Time Machine - Apple Support

For backing up that Mac (source) to another Mac computer (destination): I went to Settings on the destination Mac, selected General, File sharing, added the destination device/folder, and then right clicked on the added device/folder, and selected “Share as a Time Machine backup destination,” then clicked OK.

Then on the Mac I wanted to backup from (source), I saw the external Mac destination for doing the backup. Add the new destination, choose to use the same backup device/folder as before, and let it do the next backup.

However, it failed again on the Mini as destination: "The network disk disconnected from your Mac while backing up".

The NAS I'm using is on an AirPort Extreme (old Apple device with USB connection to a 6TB hard drive). It very recently started failing backups too with the same error message as for the Mini as a destination.

Finally, I am running Bitdefender on both the M3 MacBook Pro and M1 Mini, BUT I have disabled its protection for Time Machine backups since that previously interfered with doing backups over the LAN (several years ago now).

I have not disabled the MacOS firewall in network settings on either the MBP or Mini.

I'm going to try adding time machine to the exempted apps on firewall settings and see what happens; that would save turning firewall off and on for backing up, even though I also run an intego firewall - and have done for years without any issue for time machine. Like everyone else 15.0.1 has tripped the issue; partner's still on OS 14.6 and working fine to the same backup device.

After two failed migrations on 15.0.1 (I thought that it was my fault the first time …) I reverted the MBA M3 to Sonoma 14.7, and migrated again. TM works now, and I’ve created a usable sparse bundle from the new machine (as well as having two good ones in reserve from the old machine.

There are better ways to spend a weekend.

I have spoken to my firewall provider, and understand that 15.1 fixes the various firewall and network problems that were so freely given in 15.0.1. Lets hope that it’s soon and that they’re correct!  

At least I think I'm using AFP...I turned it on in the DS2J settings, then when connecting to an external server I typed in "afp://ds218j.local:5000" and it connected to all the shares on the DS218J. I set up Time Machine to the "Time Machine Backups" share, firewall on, and the backup is progressing...14%/28GB, five hours to go over wifi.

I may try turning off SMB on the DS218J (after the other Macs in the family do their backups) and see if it still works on my 15.0.1 MBA.

I have the same setup as you. I had to reboot twice after the 15.1 update and I started a new backup (because I thought that the old one might be corrupted). I don't know if the new backup was required but the reboots certainly were. Everything seems to be working well now. Good Luck!