Time Machine backup fails, 'not enough space' to backup "null" file

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Level 1

12 points

Time Machine backup fails, 'not enough space' to backup "null" file

When timemachine tries to backup my mac, after quite some time, it gives the error:

Timemachine could not complete the backup to xxxxx.

"<null>" could not be backed up because there is not enough space on the backup disk even after older backups were deleted.

I have MacOS Tahoe.
The Macbook disk had recently some problems that Disk util fixed after many runs under normal and safe mode.

How can I diagnose what is happening?

The "<null>" does not give a clue of where the error happened. I would expect an error that shows the problematic folder or file or containing folder or last file that was successfully backup up, so it could guide me to a part of my folder structure that has a problem.

I do not mind deleting (first copying somewhere else) a part of my folder structure that contains this corruption.

Posted on Dec 14, 2025 8:27 AM

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Answer 1

mioan Author

Level 1

12 points

Dec 14, 2025 11:43 PM in response to mioan

and here are some hints from ChatGPT

What “NULL is too large” actually means

Time Machine internally works with:

APFS inodes
Directory entries (names)
Extended attributes and hard links

This error usually occurs when one of the following is true:

A file exists whose directory entry is corrupted
An inode has:
1. no valid filename
2. or an invalid parent directory reference
A directory contains an entry with invalid UTF-8 or zero-length name
APFS metadata corruption caused by:

- interrupted writes
- kernel panic
- failed SSD block remapping
- previous filesystem errors

Time Machine cannot map the inode to a filename → it prints NULL.

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Answer 2

Dec 14, 2025 5:57 PM in response to mioan

Is this a hard disk or a NAS?

If this is a hard disk, what is the Mac storage capacity, and how bug is the Time Machine backup hard disk?

If this is a NAS, is the NAS running Samba 4.22? If so, update it to current 4.22.6 or 4.23.3 or later and test again.

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Answer 3

6x6

Level 6

19,305 points

Dec 14, 2025 5:47 PM in response to mioan

How much space left do you have on your (external) backup disk? Time Machine seems to imply that the disk is full. If it is, you'll need to acquire a new backup disk as it is no longer possible to trim existing backups.

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Answer 4

mioan Author

Level 1

12 points

Dec 14, 2025 10:52 PM in response to mioan

Thank you for your answers so far.

Some extra info:

The Macbook internal drive is 500GB.

The error is the same for two different backup destinations.

USB disk 1TB. Free space 100GB.
Synology NAS folder share 1.7GB. Free space 100GB. I will increase the quota to 2GB to give enough space for a full backup to see if this error is because of the destination drive.

If the error message is accurate, it indicates that the problem is not on the destination. It says that if older backups were removed there is still not enough space. So the file system element that is behind the "null" is bigger than 1TB, which cannot be for a macbook hard disk of 500GB.

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