Time Machine fails to back up to UniFi UNAS 2 (SMB) – “cannot back up to Time-Machine.local”

the Server drive needs to be accessible to Time Machine.

-- this may take navigating to that drive.

the shared folder needs to contain the appropriate sparse bundle disk image file

-- this is actually an entire drive inside an expandable folder .

-- mine is called GrantsMacPro.sparsebundle

when double-clicked, the sparse bundle disk image should EVENTUALLY mount, but this can take several minutes

-- one top-level item should be a folder something like "Backups of GrantsMacPro"

-- inside that folder erneath should be individual backups as of a particular date

if there is anything missing at any point, the backup will fail.

<< I can successfully connect via Finder using smb://ip_of_UNAS_drive/MacBackup >>

the instructions say you should be using the server_name, not the IP address.

Also, be sure your internet (Wi-Fi or Ethernet or Both) is set up with IPv6 set to Link-local only, NOT Off. this allows the server to be 'Discovered' by name.

I'm having a similar problem backing up to my Western Digital EX4100. It was working, but I just realized that it hasn't for two weeks now. (Fortunately I have a separate backup to an external SSD so I'm mostly OK.)

The drive is mounted and I can see the Time Machine partition in the browser, but Time Machine refuses to acknowledge its existence.

I'm updating to Tahoe 26.2 tonight. Let's see if that helps.

a search brings up this article:

https://help.ui.com/hc/en-us/articles/17299776030999-macOS-Time-Machine-Backups-on-UNAS

Tahoe 26.2 didn't help.

see if any help here:

If you can't back up or restore your Mac using Time Machine - Apple Support