Time Machine fails on new iMac M4 after migration

Question

Level 1

23 points

Time Machine fails on new iMac M4 after migration

Apologies for adding yet another Time Machine problem to the list...

I recently migrated all my data from an old (2017) iMac running Big Sur to a new iMac M4 running Sequoia. Migration Assistant did a brilliant job from a Time Machine backup on an external SSD.

The only problem is that I can't get Time Machine to work on the new iMac. An attempted backup always stops within a second or two, with the message Backup Not Completed.

I spent several hours going through an escalating series of possible solutions, guided by ChatGPT, including everything from the most basic and obvious possibilities to Terminal procedures and a full re-install of the OS, updating to Tahoe. ChatGPT eventually concluded:

"This is not:

* a configuration issue

* a disk format issue

* a permissions issue

* a migrated-user issue

* a snapshot issue

* a Sequoia-only bug

This is a Time Machine regression affecting this specific Apple-silicon iMac platform (M4) that survives OS upgrades."

Before I submit a bug report to Apple Feedback, can anyone shed any light on this, please? Is it a known issue, and could there be some simple solution I haven't tried?

iMac (M4, 2024)

Posted on Jan 20, 2026 7:54 AM

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Answer 1

Tanavino Author

Level 1

23 points

Jan 21, 2026 8:15 AM in response to den.thed

Your last suggestion worked!

I can't believe I spent so long struggling when the solution was so simple. Startup in Safe Mode, restart, connect the TM drive (a SanDisk SSD, 1TB) then add it to the list in System Settings > General > Time Machine.

Thank you very much indeed!

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Answer 2

den.thed

Level 10

94,898 points

Jan 20, 2026 8:54 AM in response to Tanavino

Did you Migrate from TM during the initial setup or did you Migrate after setting up the new iMac..?

If you Migrated during the initial setup, you should have been asked to select Use or Claim the Existing TM backups.

If you Setup the new iMac before Migration, then you may have dual accounts and one is not recognizing the TM backup.

The other problem could be that the new iMac wants to do a new complete TM backup and there is not enough space on the backup drive.

At this point, if you can not get TM to backup to the old drive:

1) Set the old TM drive aside and use a new TM drive for the new iMac.

2) Erase the old TM backup drive and start new TM backups for the new iMac.

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Answer 3

den.thed

Level 10

94,898 points

Jan 20, 2026 10:05 AM in response to Tanavino

The last thing that I have to offer is Safe Mode.

Start up your Mac in safe mode - Apple Support

Disconnect the external drive(s) > startup in Safe Mode > then restart normally and reconnect the TM drive.

If that doesn't help, give Apple Support a call at 1-800-MY-APPLE (800-692-7753)

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Answer 4

den.thed

Level 10

94,898 points

Jan 21, 2026 8:33 AM in response to Tanavino

You're welcome.

If it happens again, then I would suspect that the iMac's TB/USB port overload protection was tripped off.

Tripping the port overload can be cause by connecting to many TB/USB devices, a bad cable or a faulty external drive.

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Answer 5

Tanavino Author

Level 1

23 points

Jan 20, 2026 9:06 AM in response to den.thed

I migrated from TM during the initial setup (from an SSD - it took less than 10 minutes). I wasn't asked to use or claim the existing TM backups.

I tried three different backup drives, all with more than enough space (1TB or 2TB to backup about 250GB on the iMac).

I've tried erasing the original backups, and completely erasing the SSD or HDD. I've tried just about everything!

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Answer 6

den.thed

Level 10

94,898 points

Jan 20, 2026 9:38 AM in response to Tanavino

Hmm... the latest macOS versions require using an APFS formatted drive for TM.

Initialize an external storage device in Disk Utility on Mac - Apple Support

Back up your Mac with Time Machine - Apple Support

Also what macOS version is the new iMac currently running..?

To check, go to > Apple > About This Mac

The current stable version of macOS Sequoia is 15.7.3

What's new in the updates for macOS Sequoia - Apple Support

The other option is to go ahead and upgrade to Tahoe 26.2

What's new in the updates for macOS Tahoe - Apple Support

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Answer 7

Tanavino Author

Level 1

23 points

Jan 20, 2026 9:47 AM in response to den.thed

I've tried several drives, all APFS and GUID Partition. The OS is Tahoe 26.2.

I appreciate your input but really, I've been through all the more obvious possibilities. It does seem to something deeper and perhaps a bug.

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