Time Machine backup on macOS Sequoia cannot be completed

I, too have had problems with TimeMachine backing up to a Synology NAS server since upgrading to Sequoia (15.0 or 15.0.1, don't know which). Problem continues under 15.1.

I tried some of the other suggestions - (1) turning off Firewall, but (2)mainly removing the backup drive in TimeMachine settings, deleting my old backup folder on the NAS, booting into SAFE Mode and pointing TimeMachine settings into a newly created NAS folder share and backing up (successfully). Then doing a "normal" rebooting. Time Machine successfully backups as long as I keep my Mac Studio active. But whenever I leave my Mac and it hibernates, it always fails. But once I get back onto my Mac and it is "active", TM successfully backs up.

I have already gone into System Settings : Energy and enabled "Prevent automatic sleeping when the display is off" and disabled "Put hard disks to sleep when possible". This has not prevented the backup failures when I am away from my Mac.

I will try the suggestion above to change my power management.

I have a Mac Mini 2 with a Raid 1 WD My Cloud EX2 Ultra on my home network. Time Machine back ups worked every time until Sequoia 15.0 and same problem with 15.1. Can back up to an external HDD fine but to the EX2 Ultra is no longer possible. I have tried all the solutions offered on this forum which I am grateful for, however still no joy with EX2 Ultra. I hoping 15.2 will have a fix for this. Thanks to all for your suggestions.

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.

Update, though probably not helpful. I called Apple again tonight and learned it was an active issue, hopefully solved with next update. In the meantime, they thought it might be a lose cable to the HD - however the same thing is happening on my brand new M4 machine. I went out tonight and purchased a USBC to USBC cable to connect by backup drive directly to the USBC port on the M4. It's the first backup (since it hasn't been working), so there's an hour to go before I know if it works. The Apple rep didn't think there was much chance that would help, but I wanted to see - thinking it might be a power issue to the backup drive. I'll let you know if it works.

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?