What causes TimeMachine backups to fail on NAS devices?

Same problem with WD MyCloud NAS

Not working for me with Synology NAS and firewall off or on 🙁

Update 29th Oct: After updating to macOS 15.1, I'm able to backup to my NAS with Time Machine without issues (with Lulu and firewall enabled).

I also still have the problem after updating to 15.1.1

I back up once a week. After updating to Sequoia 15.1.1, I get the error message "Time Machine couldn't back up..." What gives???

I have never used a NAS but the experts that are far more knowledgeable than myself would probably not recommend it. I read a article about TimeMachine yesterday and a question similar to yours was asked in the comments.

I can't even restore to prior time on my Time Machine to prior update. But it says it can't do it because it says the library files can't be modified or deleted because they are needed by mac. It's useless. 2023 expensive MacBook Pro that is now absolutely useless...can't even use both monitors unless I plug them in manually AFTER I turn my computer on.

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.

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...

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.

In my case after update to 15.0.1 the "preparing to back up" step takes 30-50minutes...

Before update it was up to 3-5minutes max!

The backup step itself is quite fast as usual.

I have SMB QNAP Network share.

Disabling the Firewall , new TM setup, delete a few latest backup did not help as well...

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.

Metoo here after 15.0.1 update. Read that turning off is a temp solution. However. I have a company laptop where turning off firewall is greyed out. So no option for me.

I truly hope Apple will fix this issue. This is a serious problem!