Time Machine backup on macOS Sequoia cannot be completed

I fixed this by removing the (NAS) drive from Time Manager and adding it again. When I re-added it, I got the message "checking capabilities of drive..." and it chuntered away for a few minutes before telling me that the drive was not encrypted, was I sure? I said I was and it's trundling along nicely now.

Although I had initial success with this fix, it quit working for me after a couple of follow up back ups. I have been out of town for several weeks without access to my NAS but keeping up with all Sequoia partial updates. When I returned home I deleted my laptop's backup file on the NAS and re-ran Time Machine. After a successful backup to the NAS, the updated backups have been running just like they did pre-Sequoia. I suspect Apple has made a change in Sequoia Time Machine with one of the updates since that is all that has changed for me.

Time Machine is always a bit sluggish, but that's quite a while. How old is the Mac? Was the disk nearly full? If it has decided to clear out some old backups it would take a while on an older machine. Are there any clues in Activity Monitor?

Just to follow up. I'm still working with Apple engineers on this. There were a couple weeks of ghosting me but eventually they replied and last week logs were taken and I'm waiting for my rep to get back to me on next steps/conclusions, etc. I also updated the firmware of my Samsung T5 and T7 drives which was a challenge in itself, but did not resolve the issue so hopefully that eliminates them saying it's Samsung's issue. ;-)

More to follow when I hear back.

After two rounds yesterday with Apple support, I was no closer to getting Time Machine to work on either of my Mac Minis running 15.1.1. Afterwards, I had an idea related to a similar issue I had with a Brio webcam a few years ago. I ran to Target, picked up a quality USBC to USBC cable and connected one of my USBC drives (each computer has multiple backup drives connected to it) directly to my M4 mini and ran Time Machine. It worked. At least it worked when I tried it last night. I have more USB-C cables coming in from Amazon today and will see if that makes a difference with the other drives. I will report back here to let you know what happens.

I have the same issues with my new m4 mac mini 15.1.1 and my WD ex2 ultra. fails to backup a different random file every time. Seems like apple admit there is an issue. tiem machien worked fine on the same nas on my old mac, (big sur).

Would you post how you determine that the file not being backed up is a Notes file? The error I see on my MBP when Time Machine fails to complete doesn't specify which file is problematic.

Oops I meant MacOS 15.1

After re-adding it, trying to backup an existing volume, it has been “Preparing to backup" for about 9 hours.

OK, Nine hours is too long! Have you tried Disk Utility on the Time Machine disk?

I have the exact same issue. One on M4 mini and one on M2 mini. It’s been an hour on the phone with Apple today and we thought we made headway, but tonight it is doing the same thing over again.

I couldn’t make the Time Machine work with 15.0 or 15.0.1, so I used Terminal to add the code snipped from keithfromsalado: "sudo rm -f /etc/nsmb.conf” and restarted the Time Machine set up. It worked perfectly. Thank you, keithfromsalado!!

From completely formatting my previous backup drive did not solve the Time Machine failure to back for me.

I given up. I purchased a 3rd party option for backups. Not the same as TM, yet better than not having anything!