Sequoia Time Machine Synology NAS keeps disconnecting

'Fraid so.

I've chatted with Apple twice now.

The first time they asked me to try and start the iMac Pro in Safe Mode (failed with same error); to create a new account (failed); and reinstalling Sequoia (initially seemed to work, but then it failed overnight with the same error and has failed every time since).

I've chatted with them again and I'm waiting for a call back. First line support is saying they've not heard of this error before, so I have pointed them to this thread.

My MBA is updating completely reliably throughout.

I'll keep people informed.

15.0.1 didn't fix the problem but caused it for me. Time Machine was working fine to a USB drive connected to a Synology NAS in 15.0. After updating to 15.0.1 the problem with the "ejected or disconnected" Time Machine started. Formatting the USB drive didn't help. Only way for me to fix the problem is connecting the USB drive directly to my Mac without going via the NAS.

The issue of the firewall may be a sub-issue. I don't have the firewall on, and TM backups fail both on my synology NAS and on my WD MyCloud NAS. In my case, it appears to be linked to the Mac sleeping/locking, even if system settings=>battery=>options I have "preventing automatic sleeping on power adapter when the display is off" is checked. And on 2 different macs (laptop and desktop). No issue with Sonoma, failing backups with 15 and 15.0.1.

Well that's odd - it's started working of its own accord.

Long-time readers might remember that my iMac Pro continued to report network disconnections after upgrading to 15.1. I forgot to disconnect the drive overnight, so the Mac tried to do another backup - which worked. And it's been quietly backing up every hour since.

So I think the 15.1 update does resolve this issue. Why I got an error? No idea, but I'll be glad to never see this again.

I tried a subsequent backup with the firewall still on, after the successful backup with the firewall off, and it worked fine. Maybe turning the firewall off is a one time thing? We'll see.

Don't bother, had it since macOS 12 in combination with DSM6.x and today with macOS 15 in combination with a brand new DS923 running DSM 7.x same issue. Finder loses connection with Synology NAS (and also TM). When will Apple crushes this bug finally? In version 20? Djeezzz.

The backup is still failing even if I add Time Machine to the native firewall exception to allow incoming traffic. It will work with the firewall turned off, but once it's back on, the backup will fail.

I just started having this issue after backing up my TimeMachine to my synology NAS for years. I just upgraded to 15.1, and the issues began. This morning I upgraded to 15.2, the issue persists. The SMB mounts disconnect under any TimeMachine backup operation :/

This is a known issue with the firewall in macOS 15.0.0 and 15.0.1, which causes disconnects… I guess we’ll just need to wait for a bug fix.

Synology SMB Services beta didn't help me (with DS218+) - still "Backup couldn't be completed...", no matter whether via wifi or LAN. Time Machine on SSD connected via USB-C does however work.

Verified that the new macOS update fixed the issue.

Thanks - just checked and it was 'On' on my iMac Pro.

I've flagged the issue to Synology and their automated response suggested clearing all the SMB caches on the Synology Control Panel. But that hasn't fixed it, I've told them this hasn't worked and will see if they send a more detailed response later.

Just wondering - OP and myself are both running Sequoia on Intel processors and getting failures. My M2 MBA is backing up to the same Synology without issues.

Is anyone getting this fault on Apple processors?

I'm experiencing a similar issue. Time Machine backups occasionally fail, and file transfers to my NAS over SMB frequently cause my Macbook to disconnect from the NAS.

MacBook: Apple M3 Pro MacOS 15.0 (24A335)

NAS: DS918+ DSM 7.2.2-72806