Fix for macOS 26 Tahoe Time Machine SMB backup failing with BACKUP_FAILED_DISCONNECTED_DISK_IMAGE error on Synology, QNAP, TrueNAS, UNAS. nsmb.conf configuration guide.

nsmb.conf

[default]
signing_required=yes
streams=yes
soft=yes
dir_cache_max_cnt=0
protocol_vers_map=6
mc_prefer_wired=yes

Verified Solutions for macOS Tahoe Time Machine SMB Issues

1. Modify SMB Configuration on macOS (Most Common Fix)
   Create or edit the file /etc/nsmb.conf:

sudo nano /etc/nsmb.conf

Add the following lines:

[default]
signing_required=yes
streams=yes
soft=yes
dir_cache_max_cnt=0
protocol_vers_map=6
mc_prefer_wired=yes

Save the file (Ctrl+O, Enter, Ctrl+X), restart your Mac (optionally not necessary), and retry your Time Machine backup.

Explanation: macOS Tahoe changed the default from signing_required=no to stricter control. NAS devices with relaxed SMB settings cannot handle this without explicit configuration.

2. Remove Non-ASCII Characters from Sparsebundle Name
   If your Mac, NAS, or folder name contains non-ASCII characters (e.g., Peťov MacBook), Time Machine in Tahoe will fail:

Open Disk Utility and mount the sparsebundle

Change the name to pure ASCII characters (no accents or special characters)

Time Machine will then work normally

3. Configure Settings on Synology/NAS (Not applicable on UNAS devices)
   Navigate to Control Panel > File Services > SMB > Advanced and set:

Maximum SMB protocol: SMB3

Enable Opportunistic Locking: Yes

Enable SMB2 Lease: Yes

Enable SMB Durable Handles: Yes

Server signing: No (or "Auto")

Transport encryption: Disabled

4. Automounter Workaround (If Nothing Else Works)
   Many users have successfully used the Automounter app to keep SMB connections alive and prevent disconnections during backups.

Summary
The root cause is a compatibility issue between macOS Tahoe 26 and network NAS SMB implementations. These fixes address the SMB signing/encryption handling changes Apple introduced in Tahoe. Start with solution #1 (the /etc/nsmb.conf file) as it's the most effective and universally applicable.

FYI for Ubuntu users, updating to Ubuntu 26.04/resolute gives you a version of Samba (4.23.x) that is patched for this issue.

FYI for Ubuntu users, updating to Ubuntu 26.04/resolute gives you a version of Samba (4.23.x) that is patched for this issue.

Thank you for info.

I have a Ubiquiti UNAS Pro that is fully patched, and the above suggestion did not work for me. No odd ASCII characters in the filename. TM backups are still failing.

I have a Ubiquiti UNAS Pro that is fully patched, and the above suggestion did not work for me. No odd ASCII characters in the filename. TM backups are still failing.

It is not problem of UNAS pro but MacOS... I have UNAS Pro. Just Check If you have /etc/nsmb.conf exactly as on picture.

or try step 4

Automounter Workaround (If Nothing Else Works)
Many users have successfully used the Automounter app to keep SMB connections alive and prevent disconnections during backups.

As I mentioned, I edited the conf file per instructions above, and it did not fix. I understand it is a macOS issue. I was just stating that my setup happens to be a UNAS, just for background information to help troubleshoot.

I just resolved this by deleting my backup instance in Time Machine Settings and then trashing this plist: /Library/Preferences/com.apple.TimeMachine.plist

I saw references to a machine that I no longer own and migrated from last year in that plist. The system recreated a fresh, clean file, and I was able to continue to back up my system by creating a new backup in TM Settings.

I just resolved this by deleting my backup instance in Time Machine Settings and then trashing this plist: /Library/Preferences/com.apple.TimeMachine.plist

I saw references to a machine that I no longer own and migrated from last year in that plist. The system recreated a fresh, clean file, and I was able to continue to back up my system by creating a new backup in TM Settings.

Thank you. I was not able to reproduce your problem. This helps others.

Sorry but non of solutions 1 & 2 worked for me between a macOS 26.4 & 26.4.1 and a UNAS2 5.0.17.
I also tried the 4. Same result. :-(

Test of the 26.5 Beta. No problem anymore. End of issue.

This did not work for me. In fact, it made things worse - altho' that was partly my fault. A quick explanation: 1) I have a 2023 MacBook Pro that has done Time Machine backups to my Synology NAS with near 100% reliability for over 3 years now. 2) The 2023 machine has used Apple's AutoFS since it was bought new in 2023. 3) I recently bought a new 2026 MacBook Pro that came with Tahoe installed. 4) I used Migration Assistant to copy everything from the 2023 MBP to the 2026 MBP. 5) The 2023 MBP had to be upgraded from Ventura to Tahoe to accomodate "Migration"; the migration proceeded without a hitch (or so I thought). 6) After migrating everything from MBP 2023 to MBP 2026, I discovered that Time Machine on MBP 2026 failed repeatedly to make backups to my new and properly-configured Synology share. 7) I began casting about for solutions, and landed here; I added the prescribed /etc/nsmb.conf to the MBP 2026. 8) Now, MBP 2026 failed in new ways (e.g. refused to mount an old NetGear NAS). 9) Finally, I pulled my head out of my arse, and asked myself, "Did you actually check the contents of the /etc/autofs* configuration files?" 10) For reasons I shall never understand Apple inevitably tries their best to trash one of the /etc/autofs* configuration files; this happens repeatedly on each and every update or upgrade. It happened here despite the fact that Migration Assistant got everything else right!!

Conclusion & Moral:

In my case the solution was to delete the added /etc/nsmb.conf file, and to remove the bogus /etc/auto_master file that Apple inserted without cause or warning, and replace it with the version of the file from the 2023 MBP.
Moral: Take nothing for granted.

REF: the AutoFS files in this repo.