Time Machine Backups Failing Due to Lack of Disk

I have a 4TB external drive setup for Time Machine backups. It's actually a 4TB partition on an 18TB drive. Backups working fine for about a month, but now ...

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

There is 3.56TB free on that disk and my Mac HD is 500GB. Why is it still failing?

Thanks!

MacBook Pro 14″, macOS 26.3

Posted on Jan 11, 2026 11:38 AM

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1 reply

Jan 11, 2026 12:08 PM in response to Mark Scranton

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

your Boot disk, "Macintosh HD" is the disk that is too full.

You need to make space on the Boot drive. Time Machine needs space to stage the backup, and it is not currently available.

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Time Machine Backups Failing Due to Lack of Disk

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