Time Machine: Initial backups with dual drives markedly different in duration

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Level 1

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Time Machine: Initial backups with dual drives markedly different in duration

I have 2 new 12TB EHD (spinning) that I am using as Time Machine backups for my internal MacBook Pro M2 drive and an external SSD (total of about 4 TB of data; 3.5 on the external SSD and 0.5 on the 1TB internal Mac drive). I reformatted each drive to HFS+. I backed up Drive 1 using TM (both external and internal drive). It took about 24 hours. I then plugged in Drive 2 , and backed up using TM (both external and internal drive), which took about 4 hours. Both looked like they successfully backed up both the internal and external drive. I ran disk utility first aid on each, and they both seem fine. I'm running Sequoia 15.6.1. Should I be worried about the difference in experience between the 2 backup runs? I also can't find a BOTH button to indicate that I want to use both EHDs as backups (I see that option in many of the videos explaining how to do this). Is this no longer an option to have 2 identical (or alternating) TM backups?

Posted on Nov 16, 2025 4:32 PM

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Answer 1

Nov 16, 2025 5:30 PM in response to NancyEE

You have always been able to add additional backup drives beyond the first one. When you do, every-other backup goes to every-other drive.

Each drive is a stand-alone back that does not require the other to exist, and either can be removed whenever you like.

Disk Utility checks the drive directory, and nothing more. The function you seek is 'verify backups', but it can't be done while a backup is actively running.

Since macOS 10.14 Catalina, the default drive format for backup drives is Apple File System, APFS. But it is not backward compatible. MacOS Extended is still supported.

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Answer 2

Nov 16, 2025 6:36 PM in response to NancyEE

Another thing to keep in mind is that Time Machine does its work at low priority in the background. It generally uses 'awake but not too busy' times to do its heavy work like writing to disk.

if you were busy doing foreground work, the actual writing to disk would have been delayed.

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Answer 3

NancyEE Author

Level 1

4 points

Nov 16, 2025 6:48 PM in response to Grant Bennet-Alder

The shorter backup was during the day when I was working and the longer one when I wasn't working overnight. I will monitor the next few days to see if the time becomes similar for both drives.

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Answer 4

Nov 16, 2025 6:58 PM in response to NancyEE

it generally can't back up while your Mac is sleeping.

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Answer 5

NancyEE Author

Level 1

4 points

Nov 16, 2025 7:03 PM in response to Grant Bennet-Alder

Thanks, that might explain the difference!

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