Time Machine backup fails due to lack of space despite 1TB free on external SSD

Question

Time Machine backup fails due to lack of space despite 1TB free on external SSD

I am backing up my 2020 Macbook Pro running Mac OS Sequoia 15.2 to an external SSD using Time Machine. The Macbook has a 1TB SSD, and I am backing up to a 2TB SSD. The Macbook hard drive is mostly full (due to Photos), and I am trying to do a backup before moving the Photo Library to an external to free up space. There is nothing else on the external backup drive except the existing previous backups.

Today I connected it up and the backup failed. It fails almost immediately, and after clicking on details, the message states that it failed due to lack of space. However, the SSD itself still has 1TB of free space on it. I also thought that Time Machine would manage older files and clear some space, as needed, to manage space.

Has anyone faced this issue and found any resolution?

[Edited by Moderator]

Original Title: Macbook Pro Time Machine fails due to lack of space with 1 TB of free space

MacBook Pro 13″, macOS 15.2

Posted on Aug 29, 2025 09:22 AM

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Answer 1

Aug 29, 2025 09:39 AM in response to xtheguitarmanx

You didn't say what your storage situation is on your Mac. Apple recommends that you have at least 10% of your disk storage free. I have seen apps mess up with only 130 GB free on a 1 TB drive.

Time Machine has to analyze your drive, figure out what's changed, and massage that for inclusion in the backup. If you don't allow it to have enough storage to move around in, then it can't do its work. This applies to other apps, as well. When storage gets near full, even the simplest operations, like deleting files to free up storage, can become problematic.

Time Machine told you what the problem is, and you need to deal with that before it gets worse.

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Answer 2

Aug 29, 2025 11:41 AM in response to xtheguitarmanx

Read the error message carefully.

“Time Machine could not back up the disk "Macintosh HD" because it is nearly full.”

’It’ refers to Macintosh HD, your internal storage is too full and that’s the problem. You need to remove data from your computer’s internal storage.

Free up storage space on Mac - Apple Support

EDIT: I should have read the whole thread. But perhaps the link will help.

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Answer 3

Aug 29, 2025 09:55 AM in response to Richard.Taylor

Thanks, Richard.

To be clear, I did say what the storage situation is, if you read my post again. And that I was trying to do this backup before moving things to clear space and deal with the issue.

But yes, you are correct, I was misreading the error message, and now I clearly see that it is complaining about the Mac HD being full, not the external. Thank you for helping me realize this.

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Answer 4

Aug 29, 2025 10:01 AM in response to xtheguitarmanx

By "storage situation," I meant quantitative, like 130GB free out of 1 TB, as my example.

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Answer 5

mulberry58

Level 6

10,382 points

Aug 29, 2025 10:05 AM in response to xtheguitarmanx

Is your internal drive nearly full? Then that is the problem. Although TimeMachine backs up to the external drive, it needs some free space on the internal drive

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