User profile for user: MeansonChen
MeansonChen Author
User level: Level 1
4 points

Time Machine fails with 'unsupported disk' after Tahoe 26.2

I encountered a problem when using macOS Time Machine: the backup failed.

System: macOS Tahoe 26.2.

The backup environment is a Samba shared disk on the local area network, a standalone disk.

I have been using this environment for a period of time (about half a year), and it has always worked normally, but in the past roughly ten days I have noticed system notifications of backup failure. Clicking the notification only indicates that the backup failed, with the reason unknown. I tried multiple times and it was the same.

In Settings, I deleted this disk and then tried to add it again, but found I could no longer add it; it prompts “unsupported disk.”

There have been no other changes to the backup environment or the computer environment; I have only been using it normally. The only change is that I recently upgraded to macOS Tahoe 26.2; the previous version was 26.1.

Posted on Jan 1, 2026 12:57 AM

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User profile for user: iamshivam
iamshivam
User level: Level 2
188 points

Posted on Jan 1, 2026 11:19 PM

Tahoe 26.2 tightened Time Machine’s SMB checks and it’s no longer “best-effort” tolerant like earlier versions. Even though your config looks correct and worked on 26.1, Samba 4.17 is the weak link here. There’s a known regression where Time Machine capability advertisement over SMB breaks, so macOS now flags the share as unsupported instead of silently accepting it. Apple isn’t detecting fruit time machine anymore, not because your config is wrong, but because the SMB server isn’t responding exactly how Tahoe expects.


This lines up with reports that the issue was fixed upstream around Samba 4.22.7. Practical steps, upgrade Samba to at least 4.22.7 if possible, restart smbd and nmbd, then reboot the Mac before re-adding the disk. If upgrading isn’t an option, the only reliable workaround is exporting the disk via AFP replacement (if your platform supports it) or using a native NAS implementation that explicitly supports Time Machine over SMB. Deleting the old sparsebundle won’t help here; the rejection happens before Time Machine even gets that far. This isn’t user error and it’s not your settings drifting, it’s macOS enforcing a stricter SMB contract than your current Samba build satisfies.

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User profile for user: iamshivam
iamshivam
User level: Level 2
188 points

Jan 1, 2026 11:19 PM in response to MeansonChen

Tahoe 26.2 tightened Time Machine’s SMB checks and it’s no longer “best-effort” tolerant like earlier versions. Even though your config looks correct and worked on 26.1, Samba 4.17 is the weak link here. There’s a known regression where Time Machine capability advertisement over SMB breaks, so macOS now flags the share as unsupported instead of silently accepting it. Apple isn’t detecting fruit time machine anymore, not because your config is wrong, but because the SMB server isn’t responding exactly how Tahoe expects.


This lines up with reports that the issue was fixed upstream around Samba 4.22.7. Practical steps, upgrade Samba to at least 4.22.7 if possible, restart smbd and nmbd, then reboot the Mac before re-adding the disk. If upgrading isn’t an option, the only reliable workaround is exporting the disk via AFP replacement (if your platform supports it) or using a native NAS implementation that explicitly supports Time Machine over SMB. Deleting the old sparsebundle won’t help here; the rejection happens before Time Machine even gets that far. This isn’t user error and it’s not your settings drifting, it’s macOS enforcing a stricter SMB contract than your current Samba build satisfies.

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User profile for user: Barney-15E
Barney-15E
User level: Level 10
122,204 points

Jan 1, 2026 7:06 AM in response to MeansonChen

It doesn't sound like that is a supported backup disk: Backup disks you can use with Time Machine - Apple Support

You can sort of make it similar to a NAS if the server runs SMB3 and has enabled the "Time Machine over SMB" specification. While SMB servers that did not support that specification did work in the past, it seems Tahoe has made it mandatory. I think Samba 4.8 and later supports that specification.

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User profile for user: MeansonChen
MeansonChen Author
User level: Level 1
4 points

Jan 1, 2026 5:18 PM in response to Barney-15E

Thank you for your reply.

Yes, my SMB protocol minimum version is 3, and the Samba version is 4.17.

Below is my configuration.


vfs objects = catia fruit streams_xattr

fruit:metadata = stream

fruit:resource = stream

fruit:model = MacSamba

fruit:veto_appledouble = no

fruit:nfs_aces = no

fruit:wipe_intentionally_left_blank_rfork = yes

fruit:delete_empty_adfiles = yes

fruit:posix_rename = yes

fruit:zero_file_id = yes

min protocol = SMB3_00

max protocol = SMB3_11

kernel oplocks = no

level2 oplocks = no

ea support = yes

strict locking = yes


[TimeMachineBackup]

vfs objects = catia fruit streams_xattr

fruit:time machine = yes

create mask = 0755

directory mask = 0775

path = /mnt/DiskArray/TimeMachineBackup

valid users = TimeMachine

browseable = yes

writable = yes

read only = no

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Time Machine fails with 'unsupported disk' after Tahoe 26.2

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