Zahorone/nsmb.conf

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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.

Raw

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

Author

Zahorone commented Nov 9, 2025 •

edited

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Verified Solutions for macOS Tahoe Time Machine SMB Issues

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.

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

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

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.

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