What causes Time Machine to stall during backups?

I'm reposting the following comment for historical purposes as it was accidentally deleted by the moderators due to a miscommunication. It was originally Posted on December 29th.

Hi Folks, I bought my wife an M4 Mac mini for Christmas and have it all set up. As for background, I set it up from scratch as a new machine. It is running MacOS 15.2. In general, the new Mini is running fine. Unfortunately, she has the problem discussed in this thread. Time Machine backups cannot be completed...blah, blah, blah. Sometimes, the TM backup continues and finishes on its own once she starts to use the Mac Mini. Sometimes it doesn't and the solution is to click on "Back up now" in TM. After a couple seconds, the backup completes. I am using two external HDD as her TM Drives connected to a CalDigit Element Thunderbolt 4/USB 4 Hub. After reading through this thread, my conclusion is that there is no consistent, solid solution to this problem yet. True? Because of that, and since the "fix" for my wife's Mac mini is to simply click "Back up now" to finish the backups, I am reluctant to try all the suggested solutions only to have the problem remain. From those of you who have spent far more time on the issue, do you feel this is the most reasonable way for me to address the problem and hope that Apple eventually decides this issue is worth their time to fix? BTW, I also have a 2018 Intel Mac Mini. No problems with TM backups to its two external HDD formatted as APFS (just like the new Mac mini). It is also running MacOS 15.2.

hello mr. luigi. my displays arrive shortly so i started to disassemble my computer and peripherals including the backup drive. i feel like i got very lucky. my usb passport drive is plugged into the usb port on my 2023 m2 mini. this is sad because i shouldn't have to be lucky to have a consistent and positive backup experience. i spent more than a minute or two getting set up to make my backup drive a network drive. i'm off the hook for the nonce. however, i verify the backup every day because i use the recovery feature at least once a week. i hope apple sees this discussion string and fixes the issue as soon as possible. good luck. i'll still be following this discussion.

Hi bruiselee 13, Another update to my situation. I mentioned not wanting to pay for a different backup solution even though TM has gone rogue. Then I remembered I bought a copy of Carbon Copy Cloner for a very specific purpose years ago. Turns out it was CCC version 5 and CCC offers a 50% discount to folks who bought that version when purchasing the most recent version (CCC 7). So I said, "What the ****" and bought CCC7 at a discount. It's now installed on my wife's new M4 Mac Mini and set to backup every hour. Hopefully, this will be the most sane solution to this crazy issue. I am still using one of my drives as a TM drive set to backup once a day. All are connected to my(her) CalDigit Thunderbolt Element Hub.

This is interesting. No Spotlight and you have TM issues. Just a shot in the dark, but have you tried reindexing anyway?

i recently updated to 15.3 on both of my mac minis and my macbook. all are running ™ backups to completion. so far.

That great news, Lee! 🙌🙌🙌

Anyone download the just released MacOS 15.3.1 to see if it helps those who are still having problems with TM backups?

Same errors after getting around the Firewall issues brought in with MacOS 15.0.1 on a MacBook M1 Pro. This is a disaster of an update by Apple. The issues with Time Machine because of the update are not acceptable. Still, Apple support has not provided any solutions.

I'm curious about the solution. I've the same issue which has occurred from Sequoia 15.0.1 onwards (now running 15.1). This is on a Mac Mini 2 Pro.

I have three TM backups:

• external drive (via USB-A hub)
• external drive USB-C
• To a share on my Synology NAS

It looks like the solution involves getting the Mac into save mode and then reconnecting TM. I wasn't sure that was possible for the network share to the NAS, but if it is…

Then the big question is… when re-adding the drive into TM, what do you do with the existent TM backup, re-use it, or start again with a fresh backup?

I'm assuming here that for the NAS, the existing bundle in the share needs to be deleted from the NAS side of things?

Any further thoughts or info would be appreciated.

I'm very sorry to say but the 'fix' didn't work for me at all. I went through all the steps in full, and this morning after being out for a walk, with the Mac locked I returned to find the errors were back!

Exactly the same situation, 'some files unavailable' so the backup could not complete.

Now I'm working on the Mac (that is it is unlocked) the backups are now taking place and completing without any errors.

Apple need to fix this issue.

I meant 15.1 which is out for doing the upgrade. So, I am going to wait and see if 15.2 has the fix. I am getting tired of these constant upgrades which change settings behind my back and clearly are not sufficiently tested. Having back up issues with the firewall is not a minor inconvenience. Bad Apple.

It doesn't fix the issue.

I've been on to Apple Support most of the afternoon. Their suggestion was to wipe the Mac and restore from a TM backup.

Foolish me thought it'd also restore all the apps settings, nope...... So I'm slowly launching apps and reconfiguring them all! I'm not happy!

This probably means that the workaround of sorts (deleting the backup sparse bundle is hardly a workaround) that works for Sequoia 15.0.1 does not work for 15.1. So, between 15.0.1 and 15.1 they made the bug worse. This must be all related to the security changes they make. Too much security?

Apple needs to fix this serious bug fast now that you have found the hard way that it is worse with Sequoia 15.1.

I reported it for both 15.0.1 and yours @dbrewood for 15.1. No point of putting my case ID number as it will be edited out in accordance to Apple’s terms and conditions for using the Community site. I guess that they do not want to accept that this is, in my opinion, major as it compromises two basic functions of the OS: TM backup and firewalls. I will not update to 15.X until they fix this issue. I have two other computers around and they will remain in Sonoma now that one cannot download 15 but just 15.1 which is busted.

[Edited by Moderator]

Yep I saw that they had made the edit alas. I just checked and it seems we can't contact each other via PM on here either. You can try contacting me on Mastodon search for tekguru on the Vivaldi instance. Unless of course they edit this out too :)

I'll bite the bullet and stick with Sequoia and if needful backup manually until it is resolved. I'd class it as a critical issue they need to resolve.

I'm having this issue, or one quite similar. I get the same error message "Time Machine did not finish backing up because some files were unavailable. Backups will resume when your Mac is unlocked."

I last Friday I migrated to a new M4 Mac mini, now running OS 15.1. That went without hitch. I took over (captured? whatever the term is) the old TM backup, and that seemed fine too. Backups happen on the regular TM schedule, but sometimes I do get this error message. When I do, if I manually trigger a TM backup, that takes place without incident. I just returned home after being out for a couple of hours. Screen was locked and mini was locked; my AppleWatch unlocked the mini. I see that 20 minutes ago a TM backup too place. So this problem, whatever it is, is not even consistent for me.