Inconsistent Time Machine backups

crashmeister wrote:

The issue is that the backups are NOT alternating drives.

As of this post, all backups for Nov 03 are on TM-2 (10 of them) and ZERO are on TM-1.

I believe this can happen because the target Time Machine drive appears to not be available. See my list of candidates for how or why this can happen in my earlier reply to your post. It is usually some sort of issue (which can be intermittent) with the external drive or its cable.

For instance, some users have reported that one or more of their external drives can briefly disconnect (unmount) and then connect again, on their own. Typically a cable problem, but can be other things as well, usually hardware. Or a drive sleeps due to its settings. I use a Samsung T7 and never have seen what you are seeing. But I recall that T7 came with some proprietary drive management tools from Samsung. I erased and formatted the drive and never use such third party tools, but some have reported oddities when such tools are installed or utilized, including vendor firmware. Some Samsung drives have "one button" touch activation which requires that custom firmware or tools be installed. If you are using your T7 as a generic drive with no third party tools, then obviously this does not apply to you.

It is still November 3, can you check Time Machine Settings by inspection and see if both drives show as mounted, both should show "Next Backup: Automatic (hourly)" and then watch what happens when the next scheduled backup (one hour from the last one on either drive) should occur on the drive that is getting no backups, is there some error notification or what happens. Is it mounted and stay mounted.

I have never seen what you are reporting here, but your list of backup sets and dates does have some clues.

For instance, on Nov. 1 there is only one backup made to Drive 2, but Drive 1 seven backups. But Drive 1 has a gap in backups between 105018 and 185312 and Drive 2 has a backup at 115225. So no backups were completed to either drive between 115225 and 185312. These are hourly backups and based on the time stamps the hourly backups do not take very long to complete.

So even on the drive with more of the backups, it looks like some backups are being skipped or missed.

Some possibilities include:

• Mac is sleeping
• One or the other drive is sleeping
• I have seen some backups skipped when an intensive activity is going on, on the Mac.
• Some backups are failing. Have you seen any evidence of that? Sometimes there is a notification when that happens, or you will see "backup failed" with an explanation when checking the Time Machine Settings. You could simply leave those Time Machine Settings open in the corner of your screen to see if that happens.

Are you using any Samsung drive utilities or drive management tools? If so, their interaction with the MacOS could cause this. In fact those third party utilities might not be fully compatible with Tahoe. It is best not to use such tools, for those reasons.

Do you have any anti-virus or security tools installed? Any "cleaning" apps? Those can interfere also.

I am using Macs that either have Time Machine with two or in one case three backup drives. The only time the alternating pattern has stopped has been when one of the drives was not mounted. That has occasionally happened when the Mac, which is a laptop, is taken away after dismounting the backup drives, and then returned later in a sleeping mode, reconnected to the backup drives, and once in a while one of the drives does not mount. I realize this is not directly relevant to your setuip with a Mac Mini, but if for some reason a drive is not mounted or is sleeping, the Time Machine backup will be skipped.

crashmeister wrote:

Hopefully Apple will have a reason / solution with the next update.

In my experience, hope makes a poor plan. Barney-15E is correct, one drive is not able to successfully back up in a reliable way, and when that happens TM skips the backup. Note that your comment on the option to put disks to sleep is irrelevant since you stated you use SSDs and there's nothing to spin down.

Try running Disk First Aid on the problematic drive. Try having only that drive connected so all the hourly backups go there, and see if they're still skipped at the same frequency. Check TM settings and see if there's a red 'i' dot on the right side of a backup (I don't have any, but it would be here), it so click it to see what it says.

This one looks like classic hardware-level weirdness, not a Time Machine bug. Since you confirmed both drives work fine individually, but the problem follows the Thunderbolt port when swapped, that points straight to a flaky TB5 port or signal path, possibly a controller issue, not the drive or cable.

Time Machine just skips any drive it can’t see as mounted and won’t retry until the next cycle, so when one port drops momentarily, it’ll keep hammering the other disk.

You did the right thing getting it on Apple’s radar before the warranty expires. In the meantime, leave both drives connected but alternate them manually once a day, that’ll give you consistent redundancy until Apple tests or replaces the port hardware

If you lose one TM backup drive, you may not be able to restore a file to the most recent or second most recent version !!!

That’s not how it works. The two backup drives will have all of the same information except the very last backup where new items were added to the backup on one drive and the second drive has not run, yet.

Sounds like you have made some progress. It could be the cable or the connectors or simply something with that drive. I don't think it is some sort of bug in the OS because one drive is working fine. I think it's most likely something physical with that drive or its connection to the Mac (there is of course a connector on the Mac too).

Way back in the day, one could make copies of the Time Machine drive and proceed forward with the copy. But that is no longer possible today for reasons I don't quite understand, but apparently it is not possible to "clone" an APFS Time Machine drive and have the clone work anymore.

So if it turns out that you decide to replace that TM drive, you would start anew on the new drive and simply keep the old one as an archive of sorts.

crashmeister wrote:

I have two TM backup drives, each 4TB in size with plenty of free space (> 2 TB each) available.

TM used to alternate backups between the drives, now it seems to randomly choose which one to use.

If you lose one TM backup drive, you may not be able to restore a file to the most recent or second most recent version !!!

Drive 1 (from Oct 27 to present):

https://discussions.apple.com/content/attachment/58051f74-828d-4e12-87cc-db013737d432

Drive 2 (from Oct 27 to present):

https://discussions.apple.com/content/attachment/fe59fe4e-8934-49ed-9cb8-f7e325e28fe8

Not sure what to tell you.

You can manually alternate your backups...if it is not working as expected.

To be proactive you can file a bug report / submit your Apple Feedback here: Product Feedback - Apple

The current stable release of Tahoe including bug fixes, security updates is macOS 26.0.1

Keep your Mac up to date - Apple Support

Keep your Mac up to date - Apple Support

ref: Use Time Machine on your Mac to back up to multiple disks - Apple Support

If you lose one TM backup drive, you may not be able to restore a file to the most recent or second most recent version !!!

I interpreted that to be a general statement (which is not true) rather than a description of your own unique problem.

As of this post, all backups for Nov 03 are on TM-2 (10 of them) and ZERO are on TM-1.

From that new information you posted, TM stopped backing up to one of your two backup disks, which can occur for a number of reasons, some of which steve626 explained.

Normally, TM backups will sequentially cycle through the disks available to it at the time it backs up, and Tahoe changed nothing about that. TM will not complain about a missing one until ten days have elapsed. If and when that occurs, please write back for suggestions.

For now there is nothing for Apple to fix.

I suggest letting TM work the way it is supposed to work — automatically. If something is amiss, TM will let you know.

crashmeister wrote:

I guess you didn't read the original post well enough. The issue is that the backups are NOT alternating drives.

As of this post, all backups for Nov 03 are on TM-2 (10 of them) and ZERO are on TM-1. If TM-2 dies, there are no backups available for today.

Your description of alternating drives is incorrect.

Your problem is that it is not backing up to the other drive. That is not a problem with haphazard or random alternation.

You need to determine why the other drive is not available when it is time to back it up. If it isn't available, it will be skipped. It may be mounted and sitting there waiting, but Time Machine doesn't see it.

I can't exactly replicate your setup, my primary backup is to an NAS and my secondary backup is to a pair of 4 TB SSDs where I connect once a week then swap offsite with the other. But when I have an SSD connected, backups do alternate between the NAS and the SSD on Tahoe as they did on Sequoia.

It will be interesting to see if 26.1 corrects the issue.