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

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.