Time Machine deletes old backups and makes huge new ones

Time Machine wipes itself every couple of days.

I've been having this problem for several months now. Once or twice a week Time Machine will start a huge 20 hours long backup after which only the last 5-6 days are preserved and all the old backups are being wiped out. It started happening after I upgraded to Tahoe, but I've also introduced some other changes to the system and I have hard time identifying the cause.

I'm using Mac Mini M4 with 24GB of RAM, 512GB SSD, and 2GB external SSD connected. The time Machine disk is a regular 4TB HDD connected directly to the iMac. I'm keeping the system updated - currently it is Tahoe 26.5.1.
The internal disk has some 180GB free. Two big chunks of data is the system data (over 70GB) and the Apple Books (also about 70GB). The later is mostly an archive of books I read on my iPad, so other than downloading new purchases, it doesn't change much.
The most accessed large files on the internal disk are DevonThink databases - the biggest is 10GB and altogether they come up to slightly above 17GB - not big enough to cause huge backups, even though I'm using DevonThink daily.
The external drive holds my music library and the archives of the less accessed data. The Music folder is the area where I made the biggest changes before the backup issues have started - Because I own a lot of DRM-Free music and use non-apple DAP, I've decided to make my life a bit easier and separate once-integrated music Library. Now I have the regular Music Library folder, which holds Apple Music downloads unplayable on my DAP, and another folder with all the DRM-free music. The later is still added to the Apple Music.app and accessible without any issues - only the location of the files is different.
I've tried to exclude from Time Machine all the cache-folders and such. Currently I exclude from backups:

- /private/var/vm
- ~/Library/Caches
- ~/Library/CloudStorage
- The Time Machine disk itself
- A folder for temporary storage of files on the external drive.

Since I do use Apple Music daily, I suspect the Apple Music may be touching all the music files in the external folder, but even then it's slightly below 300GB of music and the huge backups that overwrite my entire backup history seem to contain all the local data I own (altogether it's about 1.5 TB and that's the size of the backup).

I've already tried:

Wiping the Time Machine disk clean (reformat), checking it with disk utility for errors - it seems to be fine.
Deleting Time Machine preferences.
Cleaning the caches and repairing access permissions with ONYX.

I lack the expertise to analyze the logs myself, butThe Time Machine Mechanic doesn't show any errors. The Storage display in System Preferences shows the DevonThink databases to be the largest files accessed lately, which is correct - they are still not big enough to cause the issue.

The only cloud disk I use, other than iCloud, is OneDisk. I believe I excluded its files from the Time Machine Backup, so it shouldn't be causing the problems.

I'd greatly appreciate any help with diagnosing the cause of the issue. Is there anything I should look for in the logs? Perhaps a software to verify the integrity of Time Machine databases? Or perhaps there is a way to reset the Time Machine once and for good, so that it returns to normal behavior?

Mac mini, macOS 26.5

Posted on Jun 14, 2026 5:39 PM

Top-ranking reply

woodmeister50

Level 7

21,873 points

Posted on Jun 15, 2026 4:58 AM

One thing to try, first start by tracking daily disk activity of "backup" in activity monitor. Since it is a running total, you will need to record each day's values, read and particularly write, and subtract from the previous day. A simple Numbers or Excel spreadsheet can easily be set up for this.

This will establish a base line of activity and try to roughly compare the results to the amount of changes you made during the day. If the backupd shows a lot more activity than what you believe you have changed the next step I would take is to exclude the external drive from Time Machine backups and continue to monitor backupd activity. This is a process of elimination as to what could be the source. Note that you will need make alternate backups of any irreplaceable data on that drive. You may only need to do this for a day or two if backupd if its daily activity drops dramatically.

If no change in backupd activity after doing this, then try to exclude your internal drive and monitor results as the previous test.

If either of the above have a huge effect, then you may need to narrow things down by excluding individual folders to try and find the "trigger".

If neither of the above tests causes any change, then there could be a corrupted Time Machine database file or preference that may be causing Time Machine to make excessive changes. But worry about that if it comes to that.

Side note, any reboot or shutdown will reset Activity Monitor statistics so you will need to account for that in your tracking.

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24 replies

John Galt

Level 10

162,694 points

Jun 15, 2026 7:59 AM in response to Krzysztof Zabko-Potopowicz

Any perceived hostility is yours. "Junk" is not hostile; it's just a statement of fact.

State institutions in some countries don't understand not needing anti-virus software.

Who owns that Mac? Whose data does it contain? Who would suffer from its loss? I surmise you and I both know the answers to those rhetorical questions. Proceed accordingly.

Jun 15, 2026 9:09 AM in response to Barney-15E

The only cloud storage I have connected to MacOs is the OneDisk. The data I store there is either temporary of I have duplicates elsewhere, so I don’t need backups.

The reason I’d excluded the folder was, I suspected OneDisk to be the app that touches all the files when it syncs… but I guess it’d need full disk access to do that. In process of hunting for the culprit I disabled permissions to access disk or to run in the background for multiple applications, but I still haven’t found the culprit.

The backups seem not to be full backups, but some of them seem to be the size of almost all my local data. I’m at a loss what could be doing this. After reading some of the replies, I need to check if it isn’t only the external thunderbolt SSD that is getting backed up whole every few days.

Barney-15E

Level 10

125,828 points

Jun 15, 2026 7:05 PM in response to Krzysztof Zabko-Potopowicz

The backups seem not to be full backups, but some of them seem to be the size of almost all my local data.

That's because it should seem to be all of the data. The snapshots are an entire duplicate of the drive, but that doesn't mean that it is all copies of the data. Data that has not changed is referenced to the original storage location. Those references are indistinguishable from the original file.

Time Machine is nothing like anything before it. Don't expect it to look like what you think it should look like.

I think some other backup software uses similar schemes, but I'm not certain.

Jun 18, 2026 7:32 PM in response to Barney-15E

I'm sorry for the delay in replying to your post.

My Time Machine is set to backup every hour since always. Obviously there are gaps when the machine sleeps at night or when I shut it down, but most of these backups are incremental and very small (few hundred gigabytes each time).

I understand the backups are not full backups now, but it seems a large chunk of my external NVMe M2 drive (connected via TB3 case) is getting fully backed up every couple of days.

I have not installed anything major in years. The last big piece of software installed was Affinity 2, which I haven't upgraded to the newest version yet and even that was before buying this Mac Mini. I'm using Word, Apple Music, DevonThink and Safari daily, as well as some minor dictionaries, Notability and such. I'm syncing with iCloud and OneDrive, but that also hasn't changed for years.

The only things that come to mind are me rearranging Apple Music Library (not big enough to cause that much data being copied though) and Mac OS Tahoe - specifically I think the first big backup started directly after patching the system - I didn't think much of it at the time, because it was right after the update and the backup had some right to be bigger than usual. I tried to look at system activity through the last couple of days, but I can't say there's anything unusual.

John Galt

Level 10

162,694 points

Jun 18, 2026 8:11 PM in response to Krzysztof Zabko-Potopowicz

Cleaning the caches and repairing access permissions with ONYX.

I just noticed that.

If you want TM to work, stop messing with it. Turn it on and forget about it. That is how Apple designed it to work.

Forget about using things like "Time Machine Mechanic" as well. It initially sounded like a great idea, so I tried it, and its report was not accurate. For example it reported that no backups were completed in any time frame I provided. Since that was obviously false, my (already low) confidence in it diminished even further. It's irrelevant. Nice idea though.

You can examine Time Machine's logs yourself. I use a shell script to do that, which has since been copied all over this site. It's not nearly as convenient as using "Time Machine Mechanic" or things like it, but at least its results are correct.

If you want your Mac to work, then let it work.

... most of these backups are incremental and very small (few hundred gigabytes each time).

A few hundred gigabytes each time is not small, and will take hours to complete. I suspect that's not what you intended to write.

Jun 18, 2026 8:43 PM in response to John Galt

Well, I had let the system just go for over a month, but since Time Machine wasn't fixing itself and continues to do these huge backups every couple of days, I've started looking for tools that may help me understand the source of the problem.

Time Machine Mechanic has been useful in helping me understand only the external drive is producing these huge backups, so it wasn't entirely useless... but I can't get any further insight from it.

Jun 18, 2026 8:48 PM in response to MrHoffman

MrHoffman wrote:

Do we have Little Snitch or other “haxie”-like apps installed here, too?

And was “It actually crashed once” the backup crashing, or the Mac crashing?

No such apps that I'm aware off. There are iStat menus which I had used to monitor temperatures in the past, and recently I've been running them to see if there's any abnormal disk traffic, but I don't think these mess up in any way with the system.

As for the crash, when I came back from my lunch break the Time Machine disk was unmounted and the control panel indicated unfinished backup. I re-mounted the disk (Disk Utility couldn't see it, I had to unplug and plug it again) and after system checking up the drive, I let the backup restart on its own. It finished without problem, but as you can see from the screenshot I've posted yesterday, the PID of the backupd task has been completely different since the event - therefore I assume the demon must have crashed.