Time Machine doing full backups on macOS Tahoe

My timemachine is not doing incremental...

I have a Macbook pro with MacOS Tahoe 26.2.

I am using Timemachine since several years, but strangely since a couple of months timemachine seems to have become crazy. It seems not to do incremental backups anymore, but full backups each time.

As an example, I have around 650 GB of data that I want to backup to a 2 TB NAS share on my QNAP NAS. Each backup tackes nearly 650 GB. With 3 backups I am done... The share becomes full and backups fail.

My understanding is that timemachine should do incremental backups and backup only new data between the two schedules, so a couple of GB per instance at max. And on another hand, when the share is full, it should clean the storage to reclaim space.

I am not sure that this is related to SMB or the QNAP NAS as I made the test today with a 2 TB USB drive, formatted with APFS, and the behaviour is the same... 3 backups and it's full and timemachine fails.

I tried deleting all my backups, recreated the share, deleted snapshots etc... but it's still failing.

Do you have any idea ?

Thanks for your help

MacBook Pro 14″, macOS 26.2

Posted on Jan 6, 2026 9:24 AM

Top-ranking reply

Tesserax

Level 10

160,984 points

Posted on Jan 6, 2026 10:27 AM

When Time Machine keeps doing full backups, it’s usually because it believes each backup is going to a “new” destination. This can happen if macOS can’t reliably recognize the backup disk, the volume identity keeps changing, snapshots are corrupted, or the backup history database is damaged. Network shares can trigger this, but since it also happens on a local APFS drive, it points more toward a Time Machine state or snapshot issue on the Mac itself.

To resolve this, I’d suggest a clean reset of Time Machine’s local state and snapshots, as follows:

First, turn off Time Machine.
Then open Terminal and run tmutil listlocalsnapshots / << This will provide you with a list of all current snapshots on your Mac. You should see something like: com.apple.TimeMachine.2026-01-05-100421.local
Then remove them using tmutil deletelocalsnapshots <date>.
After that, delete the Time Machine preference file (/Library/Preferences/com.apple.TimeMachine.plist).
Reboot the Mac, re-enable Time Machine, and select the backup disk again.

Alternately, for steps 2 & 3, you can use the Disk Utility instead. From the Disk Utility select the "Data" volume. A list of APFS snapshots should appear at the bottom of the utility. If they don't, from the Utility's menubar, select View > Show APFS Snapshots. You can then select each snapshot and delete them.

If the issue persists, try disabling Spotlight indexing on the backup disk, make sure the drive isn’t being mounted under different names, and confirm the disk has at least 2–3× your data size so Time Machine can rotate snapshots properly. Finally, we can look into the Mac's system logs to see what Time Machine is (or not) doing to cause this.

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31 replies

Feb 20, 2026 10:11 AM in response to John Galt

Thanks John, you are correct - I have re-activated it and Time Machine works still. I don't know why it's randomly just started working properly in that case, I have had this issue since MacOs 26.0

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Time Machine doing full backups on macOS Tahoe