What causes Time Machine to stall during backups?

Try re indexing Spotlight. It fixes it for a few of us. For me wife 4 weeks, friends 3 weeks, myself had to reindex after two weeks.

Open Activity monitor when it does. Look at CPU and memory columns. Click on the header to sort. You can see what is hogging the computer. ;)

You still have the Router firewall (most come with protection software, including a Firewall).

The TM back up will complete normally if you turn of the Mac OS firewall. Otherwise, you will continue to have issues.

Well my TM machine backup failed again tonight. Reindexing is not working for the long term. But a restart will always get it working again. Just installed OS 15.1.1, which included the usual restart. TM working again and will see how long it keeps working now.

Hi bruiselle, Sadly, just connecting one backup HDD to one of the Thunderbolt 4 ports in the back of the Mini did not solve the problem. So, for me, directly connecting the backup hard drives to the Mini is not a solution. I could switch to Carbon Copy Cloner, but I really don't need its extensive functionality and it seems a shame to pay for a pretty expensive application to solve this problem. CCC is usually set to backup once a day and what I may do is steal that idea and apply it to TM for now. Instead of backing up every hour (not really necessary in my case) I think I will set it to back up "Everyday" and when I am usually on my computer. (I assume it sets the time of the day when it will backup to the time the user initiates the "Everyday" option.) This will hopefully mitigate the annoyance of manually using "Back up Now" because the problem only occurs when the Mini is locked. The good news is I have all the ports on my Mini back. Since there was no difference in performance between direct connection to the Mini and connecting my backup drives to my CalDigit Thunderbolt Element hub, I reconnected the two backup hard drives to the Element hub. Thanks so much for going along with me on this adventure and trying to help.

If this comment appears somewhere random in the discussion, it was posted on Feb. 5.

I am subscribed to a Reddit discussion on this same issue. One of the participants just posted that after updating to the latest version of MacOS, 15.3, his TM backups have proceeded without issues. Has MacOS 15.3 fixed the problem for anyone here?

i updated my MacBook Air to MacOS 15.3 two days ago, and so far the backups have been completing on schedule without issue. My schedule is only once per day though. I have forced a couple of backups since updating. and they completed successfully too.

Before the update, I was changing Lock Screen settings to keep the MacBook from locking or sleeping during backups, and I kept it powered during the backup. Those tricks worked most of the time. This morning, I noticed that a backup completed overnight with no special Lock Screen settings, and the laptop was closed and only on battery power. Under these conditions I would have fully expected any overnight backup to fail, so when it didn’t, I was stunned. Maybe there’s something to 15.3? I’m cautiously optimistic but still skeptical. Fingers crossed, but time will tell I guess.

Update, sent the following through to Apple Support:

..............as you know I've been rebuilding the Mac slowly and logging what

changes I've made. All has been well with the TM backup problem not

coming back, until this morning when it reappeared. When the Mac was

unlocked this morning I got the usual 'Backup Not Completed

Time Machine couldn't back up to "TM Extreme SSD as files were in use......' message.

The only changes I'd made since all was well was to add the 'Home'

and 'Notes' widgets to the desktop. The triage this further I?'m now

removing the 'Home' widget and seeing if that fixes the problem. If it

is still there then I'll look to remove the 'Notes' widget.

Other changes that were made included Brave browser extensions, so I'll look at those on a secondary basis.

Fingers crossed......

For me since fixing the firmware 'issue', the 'problem' has only come back once. I reported to Apple:

I'm sorry to say that the error returned yesterday after 37 Days, 5 hours, 3 minutes of no issues.

I then carried out:

- Quit out of Apple Books which I'd started using again earlier in the day, and had imported some ePub books

- Reformatted an external derive I use for Carbon Copy Cloner backups on some files from my NAS, as CCC reported the drive was full.

- The 'Use iPhone Widgets' on the Mac was turned off again. After the latest MacOS update it had turned the option back on.

- Emptied the rubbish bin

- Rebooted the Mac

Everything seems to be backing up normally once more.

Since that 'reset' it's now been running fine for 22 days 10 minutes. I've been sending regular status emails to Apple support, but despite that it looks like they may have closed the ticket out as links to it no longer work. I'll be chasing them next week.

In either case, you need to erase the previous backups. Must start fresh.

I have been fine for 3 days now, performing hourly backups wit OS 15.1.1

Please, provide Apple with documentation of your experience via Apple Feedback. Here is the link to Apple Feedback for the Mac Mini. Feedback - Mac mini - Apple

This procedure worked fine.

I also tried another one with my NAS: I rebooted the Server and the Mac and requested another back-up. For a while the error message for failed backups persisted but when opening the TM Settings, I noticed that the NAS disk showed the time of a backup without a warning attached to it. After the second or third backup to the server, no failed backup messages showed up. It seems that it is all is well now.

..... And I can confirm the issue was fixed after the restore, but that was on macOS Sonoma, as soon as I uplifted to Sequoia the issue returned, as I expected.

I've just spoken to Apple Support again and she is going to investigate and cal me back tomorrow.

If it is any use to anyone my case is 10*******93.

If you have reported the case and have a case number please feed it back, as she did indicate she was not aware of any other reports. If you have not reported it please do so and feel free to reference my case number.

The more of us that report it the quicker it will get resolved.

[Edited by Moderator]

It happened to me. Time Machine failed to back up. Then it deleted my entire two year history of backups.

Time Machine cannot be trusted. This isn't the first time it deleted years of backups.

I've removed it, cleaned up its remnants (like the hidden snap shots consuming about 100gb), and reformatted my backup storage.

Now I'm using Carbon Copy Cloner. It's great. It's what Time Machine promised to be, and a lot more.